Gas-phase functionalization of surfaces including carbon-based surfaces

ABSTRACT

The invention provides methods functionalizing a planar surface of a graphene layer, a graphite surface, or microelectronic structure. The graphene layer, graphite surface, or planar microelectronic structure surface is exposed to at least one vapor including at least one functionalization species that non-covalently bonds to the graphene layer, a graphite surface, or planar microelectronic surface while providing a functionalization layer of chemically functional groups, to produce a functionalized graphene layer, graphite surface, or planar microelectronic surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of prior application Ser. No. 11/703,375,filed Feb. 7, 2007, issued as U.S. Pat. No. 7,767,114, Aug. 3, 2010,which claims the benefit of U.S. Provisional Application No. 60/766,000,filed Feb. 7, 2006, the entirety of both of which applications arehereby incorporated by reference. This application claims the benefit ofU.S. Provisional Application No. 60/934,454, filed Jun. 13, 2007, theentirety of which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.CTS-0236584 awarded by NSF. The Government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

This invention relates to techniques for forming layers of material on asurface of a structure, and more particularly relates to techniques forfunctionalizing surfaces, including surfaces of carbon-based structures,to enable the formation of layers of material on a structure surface.

In the fabrication of electronic and microelectromechanical systems, aselected coating material or coating process can be found incompatiblewith a chosen substrate or material structure on which the coatingmaterial is to be formed. For example, atomic layer deposition (ALD) ofa coating material on a structure surface requires that the structuresurface be reactive with the ALD precursor molecules required forproducing the selected coating material. But for many material systems,a chosen substrate or material structure may be chemically inert to therequisite ALD precursor molecules for producing a desired material layeron the structure.

For example, this condition is true in general for carbon-basedmaterials such as graphite, graphene, carbon nanotubes, and otherfullerene structures. Such materials are characterized by a compositionmainly of ordered hexagonal groups of carbon atoms or hexagonal andpentagonal groups of carbon atoms. For example, graphene is a planarlayer of carbon atoms that are bound together in a hexagonal networkthat is one atom thick. Graphite is a bulk structure composed ofmultiple stacked layers of graphene. Carbon nanotubes can be consideredas graphene sheets rolled into nanoscale cylinders. Buckyballs and otherso-called fullerene structures can be considered as layers of graphenerolled into nanoscale spheres or other three-dimensional structures.

In general, the carbon surfaces of these structures are chemically inertto a wide range deposition species, including many ALD precursormolecules. As a result, many materials cannot be deposited or otherwiseformed on the surfaces of these structures. But the unique physical,electrical, and chemical properties of these carbon-based structures areof particular importance for next-generation electrical and opticalapplications and for many nanoscale systems.

For example, carbon nanotubes are being employed increasingly for a widerange of nanosystems and nanodevices. The unique electronic structure,exceptional elastic properties, and extremely high aspect ratio allcharacteristic of carbon nanotubes address many considerations that arecurrently of interest for nanosystems and nanodevices. In themicrofabrication of such carbon nanotube-based systems and devices, itcan be desirable or required to deposit one or more layers of materialon a nanotube surface. Coaxial coating of a carbon nanotube, such thatthe longitudinal coaxial surface of the nanotube is substantially fullysurrounded by one or more coated layers, can be particularly desirable,for, e.g., providing a coaxially symmetric nanotube structure andmaterial properties. Surround-gate transistors and other deviceconfigurations are particularly reliant on such a coaxial coatingarrangement. Further, suspended carbon nanotube-based device geometries,often employed for sensing applications, generally are preferablyimplemented with a coaxially-coated nanotube.

A nanotube wall configuration, electronic structure, and surfaceproperties can all impact the ability to deposit a selected material onthe surface of a nanotube. For example, uniform, conformal coating ofmulti-walled carbon nanotubes (MWNTs) can under some conditions beaccomplished by atomic layer deposition (ALD) or by chemical vapordeposition (CVD). ALD allows for the deposition of a wide variety ofmaterials at relatively low processing temperatures with superiorthickness precision and high composition uniformity, and therefore is anattractive deposition technique for many applications. By enabling sucha high degree of deposition precision, ALD well-addresses carbonnanotube nano-scale geometries and overcomes the limitations of CVDdeposition techniques. But for many applications, it is understood thatMWNTs can be coated by an ALD process only because MWNTs arecharacterized by the existence of defects, at nanotube surfaces, thatcan act as nucleation sites for ALD precursors. MWNT ALD coatingstherefore cannot be guaranteed to be reproducible or uniform.Single-walled carbon nanotubes (SWNTs) are characterized by a much moreideal and defect-free surface structure than MWNTs. SWNTs are found tobe chemically inert to ALD precursor molecules. As a result, continuousALD coating onto a SWNT by ALD is not in general conventionallyachievable for any process conditions.

Other similar scenarios exist in which a selected coating material orcoating process is found incompatible with a chosen nanotube wallstructure or other configuration. For example, chemical vapor depositionprocesses in general cannot be guaranteed to produce a uniform nanotubecoaxial coating, and can require plasmas or deposition temperatures thatare so high as to impact the electrical or mechanical properties of ananotube. For many process conditions, SWNTs are chemically inert to CVDprecursors at CVD temperatures less than about 400° C. Although physicaldeposition methods (PVD), such as sputtering or evaporation, cansometimes deposit metals or other materials directly onto SWNTs, suchdeposits are not conformal, and do not uniformly surround the tubes dueto, e.g., the directional nature of the methods. A “conformal” coatingon a nanotube is here meant to refer to a coating that wraps completelyaround the nanotube with uniform thickness on all sides. It is thereforedifficult to reliably and benignly make conformal layers of materialaround a nanotube.

For many applications, it is desirable to coat a nanotube uniformly andconformally with insulating and metallic materials in the formation ofan electronic device such as a coaxially-gated nanotube transistor. Butin general it can be difficult to uniformly and conformally coat ananotube, and particularly a SWNT, with selected materials. As explainedabove, SWNTs are inert to ALD precursors and therefore cannot be coatedby an ALD process. CVD and PVD techniques do not reliably produce athin, uniform and conformal layer on a nanotube. Liquid-chemicaldeposition methods are known to enable the coating of SWNTs with SiO₂,which is a low-κ dielectric, but do not enable the deposition ofhigher-κ materials on a nanotube.

It has been suggested that to overcome this difficulty, a nanotubesurface can first be functionalized to render the surface susceptible todeposition precursor molecules, thereby to enable deposition by aselected technique such as ALD. For example, a liquid-based techniquecan be employed for functionalizing a SWNT surface by covalent chemicalbonding of a functionalization layer to the SWNT surface. The resultinglayer provides functional groups, covalently bonded to the nanotubelongitudinal sidewall, that are reactive with deposition precursors suchas ALD precursor molecules.

While this technique indeed enables uniform ALD film deposition oncovalently functionalized SWNT surfaces, the liquid-based process isprocedurally tedious and could be impractical for large-scalefabrication scenarios. Furthermore, the covalent nature of the chemicalbonding process can in general change the hybridization state of ananotube, destroying optoelectronic and/or other properties of thenanotube. A post-functionalization heat treatment or other process canbe required to recover the initial hybridization state of such afunctionalized nanotube. As a result, it has heretofore been impracticalto employ covalent functionalization as a means for enabling uniformdeposition of a selected material, and particularly an insulatingmaterial, on a carbon nanotube.

This condition applies in general across the family of carbon-basedmaterials and structures including graphene, graphite, buckyballs, andfullerene structures other than carbon nanotubes. Covalentfunctionalization of such carbon-based structures is for manyapplications detrimental and/or not possible within the limitations of agiven microfabrication process sequence. The technical potential forsuch structures in microelectronic and nanoscale system applications istherefore limited by an inability to form layers of selected materialson the structures.

SUMMARY

The invention overcomes the limitations of covalent-bonding,liquid-based surface functionalization techniques by providing afunctionalization method that requires only weak, physical bonding, suchas non-covalent bonding, to produce a functionalization layer on thestructure.

In one example process provided by the invention, a carbon nanotubesurface is exposed to at least one vapor including at least onefunctionalization species that non-covalently bonds to the nanotubesurface, to form —CH₃ functional groups at the nanotube surface. Thefunctionalized nanotube surface can then be exposed to at least onevapor stabilization species that reacts with the functionalization layerto form a stabilization layer that stabilizes the functionalizationlayer against desorption from the nanotube surface while providingchemically functional groups at the nanotube surface to produce astabilized nanotube surface. The functionalization and stabilizationlayers can be characterized as substantially one monolayer in thickness.The stabilized nanotube surface can then be exposed to at least onematerial layer precursor species that deposits a material layer on thestabilized nanotube surface.

This enables production of a single-walled carbon nanotube that isconnected between electrically conducting source and drain contacts,with an insulating layer of a first thickness disposed on at least aportion of the source and drain contacts and covering a span of thenanotube, at each end of the nanotube at the source and drain contacts,that is substantially equal to the first thickness. Then a gatedielectric layer of a second thickness, less than the first thickness,can be disposed coaxially around a central span of the nanotube on whichwas not disposed the insulating layer of a first thickness, and anelectrically conducting gate metal layer can be disposed at leastcoaxially around the dielectric layer on the nanotube.

The processes of the invention extend to methods for functionalizing aplanar surface of a graphene layer, a graphite surface, ormicroelectronic structure. Here, the graphene layer, graphite surface,or planar microelectronic structure surface is exposed to at least onevapor including at least one functionalization species thatnon-covalently bonds to the graphene layer, a graphite surface, orplanar microelectronic surface while providing a functionalization layerof chemically functional groups, to produce a functionalized graphenelayer, graphite surface, or planar microelectronic surface.

After functionalizing a structural surface in accordance by exposing thesurface to at least one vapor including at least one functionalizationspecies that non-covalently bonds to the surface while providing afunctionalization layer of chemically functional groups, to produce afunctionalized surface; the functionalized surface can be exposed to atleast one vapor stabilization species that reacts with thefunctionalization layer to form a stabilization layer that stabilizesthe functionalization layer against desorption from the surface whileproviding chemically functional groups at the surface, to produce astabilized surface. Then the stabilized surface can be annealed at apeak annealing temperature between about 300° C. and about 700° C.

The functionalization, stabilization, and annealing steps of theinvention can be applied to a wide range of substrates andthree-dimensional structures, such as carbon-based structures likecarbon nanotubes, other fullerene structures, planar and non-planarcarbon-based materials, and various microelectronic structures. Theinvention is well-suited for enabling material deposition on anythree-dimensional structure, planar or non-planar, that is inherentlyinert to such deposition and that is compatible with thefunctionalization and stabilization processes.

Other features and advantages of the invention will be apparent from thefollowing description and accompanying figures, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic side view of a carbon nanotube suspended over atrench for functionalization in accordance with the invention;

FIG. 1B is a schematic planar view of carbon nanotubes disposed on asurface for functionalization in accordance with the invention;

FIGS. 1C-1D are schematic side and planar views, respectively, of acarbon nanotube suspended over a through-hole in a support structure forfunctionalization in accordance with the invention;

FIG. 1E is a schematic view of carbon nanotubes disposed vertically froma substrate for functionalization in accordance with the invention;

FIG. 1F is a schematic view of a carbon nanotube suspended from the edgeof a beam for functionalization in accordance with the invention;

FIG. 2 is a schematic diagram of the reaction mechanism for Al(CH₃)₃with NO₂ adsorbed on a single-walled carbon nanotube surface in thefunctionalization process of the invention;

FIG. 3A is a plot of measured conductance of a SWNT as a function oftime as NO₂ is adsorbed and desorbed from the surface of the SWNT;

FIG. 3B is a plot of measured conductance of a SWNT as a function oftime as Al(CH₃)₃ is adsorbed and desorbed from the surface of the SWNT;

FIG. 3C is a plot of measured conductance of a SWNT as a function oftime as ALD functionalization cycles of NO₂ and Al(CH₃)₃ dosing arecarried out;

FIG. 3D is a plot of measured conductance of a SWNT as a function oftime as one ALD functionalization cycle of NO₂ and Al(CH₃)₃ dosing iscarried out;

FIG. 4A is a plot of measured functionalization layer thickness as afunction of number of ALD functionalization cycles of NO₂ and Al(CH₃)₃dosing;

FIG. 4B is a plot of measured functionalization layer thickness as afunction of NO₂ dosing in an ALD functionalization cycle of NO₂ andAl(CH₃)₃ dosing;

FIG. 5A is a plot of measured conductance of a SWNT as a function oftime as one pulse of NO₂ dosing is adsorbed and desorbed from thesurface of the SWNT where the SWNT Schottky barriers with electrodes atthe SWNT ends have not been passivated;

FIG. 5B is a plot of measured conductance of a SWNT as a function oftime as one pulse of NO₂ dosing is adsorbed and desorbed from thesurface of the SWNT, where the SWNT Schottky barriers with electrodes atthe SWNT ends have been passivated;

FIG. 5C is a plot of measured SWNT conductance as a function of time asmultiple NO₂ pulse dosing is carried out, indicating the conductance fora SWNT having Schottky barriers with electrodes at the SWNT ends thathave not been passivated and are exposed, and for a SWNT having Schottkybarriers with electrodes at the SWNT ends that have been passivated andare coated;

FIGS. 6A-6I are schematic views of process steps in the fabrication of adoubly-suspended carbon nanotube field effect transistor in accordancewith the invention;

FIGS. 7A-7M are schematic side views of the process steps in thefabrication of a vertical carbon nanotube field effect transistor inaccordance with the invention;

FIGS. 8A-8B are plots of capacitance as a function of applied voltagefor MOS structures formed on a Si substrate that does not include afunctionalization layer and on a Si substrate that does include afunctionalization layer, respectively; and

FIGS. 9A-9F are plots of capacitance as a function of applied voltagefor MOS structures formed on a Si substrate and not annealed, annealedat 300° C., annealed at 400° C., annealed at 500° C., annealed at 600°C., and annealed at annealed at 700° C., respectively.

DETAILED DESCRIPTION OF THE INVENTION

The functionalization processes of the invention can be applied tosubstantially any structure having a surface chemistry that isattractive to the functionalization species, as described in detailbelow. Once functionalized, vapor or liquid processing at the surface ofa structure can be carried out to form one or more layers of material onthe structure. Example structures to be functionalized in accordancewith the invention include planar microelectronic material substrates,e.g., wafers, pieces of substrates, chips, or other planarmicroelectronic structures having a surface chemistry that is compatiblewith the functionalization chemistry. Layered, composite, or matrixmaterials and structures or other planar microelectronic structureshaving surfaces to be functionalized can also be processed. Silicon,silicon dioxide, silicon nitride, glasses, metals, and other insulating,semiconducting, and conducting microelectronic materials can befunctionalized in accordance with the invention. The material structurescan be planar, as well as coaxial, spherical, or some other selectedgeometry, and can be provided as a microelectromechanical ornanoelectromechanical structure or system. In general, any structure forwhich there is required a surface to accommodate material formationand/or deposition can be functionalized, if the surface is compatiblewith the functionalization chemistry.

Considering the important class of carbon-based structures, anystructures having carbon surfaces, including bulk graphite, graphene,carbon nanotubes, buckyballs, and other fullerene structures thatexhibit inertness to precursors for ALD or other material deposition,formation, or growth process can be functionalized by the processes ofthe invention to overcome such inertness. Carbon nanotubes present animportant example of a carbon-based structure for whichfunctionalization is required. For clarity, the following descriptionfirst presents an example functionalization process directedspecifically to carbon nanotubes, but the invention is not limited tosuch, and further examples of alternative structure functionalizationare presented later in the description.

The functionalization process of the invention can be carried out on acarbon nanotube of any selected wall configuration, includingsingle-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), andcan be carried out on a nanotube of any selected electrical state,including metallic or semiconducting nanotubes. For any of theseconditions, the functionalization process of the invention produces afunctionalization layer that is physically bonded to the outside surfaceof a nanotube in a benign manner that avoids chemical modification ofnanotubes, thereby preserving the electrical, optical, and mechanicalproperties of the nanotube. While not chemically bonded to the nanotubesurface, the functionalization layer of the invention provideschemically-active functional groups at the nanotube surface that arereactive to deposition precursors for depositing a material layeruniformly over the functionalization layer. The functionalization layerof the invention thereby preserves the nanotube properties whileproviding a nanotube surface condition that facilitates subsequentdeposition of a uniformly thick film or films all over the surface ofthe nanotube.

Once a carbon nanotube surface is functionalized in accordance with theinvention, vapor or liquid processing of the nanotube can be carried outto deposit one or more layers of material on the nanotube in a precise,reliable, conformal manner. ALD techniques, chemical vapor deposition(CVD) techniques, plasma deposition techniques, other physicaldeposition techniques, and solution-based deposition techniques can allbe employed to produce on a functionalized carbon nanotube a desiredconfiguration of a material layer or layers. Example functionalizationprocesses provided by the invention and example material depositionprocesses are described in detail below.

For many applications, it can be preferred to deposit a material on ananotube in a manner that coaxially coats the nanotube surface, i.e.,that surrounds at least a portion of the length of the nanotube. Toachieve this condition, it is accordingly preferable to substantiallycompletely coaxially coat a nanotube with a functionalization layer ofthe invention, along substantially a selected portion or the entirelongitudinal sidewall length of the nanotube. This can be accomplishedconveniently if the nanotube to be coated is synthesized or placed in anarrangement that provides exposure of the nanotube sidewallcircumference along some or all of the nanotube length.

FIG. 1A is an example of such a configuration. Here, a nanotube 10 hasbeen synthesized between two catalyst regions 12, 14 across a trench 16provided in a substrate structure 18. The suspension of the nanotubeacross the trench provides complete access to the nanotube sidewall,thereby enabling the formation of a functionalization layer around theentire nanotube. In contrast, in an arrangement like that shown in FIG.1B, nanotubes 10 that are synthesized lying flat on a substrate 18 arenot fully exposed to the ambient, and cannot in one step be fully coatedwith a functionalization layer.

In accordance with the invention, nanotubes that are synthesized in theconfiguration of FIG. 1B can be manually manipulated to be arranged in asuspended configuration like that of FIG. 1A, or other suitableconfiguration, for enabling full coating. For example, nanotubes can bemoved to positions over holes or trenches by flowing after growth inanother selected location. Alternatively, the nanotubes can bemaintained flat on a surface and multiple coating cycles be employedwith manual rotation of the nanotubes between each cycle.

Turning to FIGS. 1C-1D there is shown a second example of a suspendednanotube configuration. In this example, a nanotube 10 has beensynthesized between two catalyst regions 12, 14 across a through-hole 20in a substrate 18. As shown in FIG. 1D, the through-hole can be square,or can be of another selected geometry, either straight or curved. Thetrench 16 of the example of FIG. 1A likewise can be of any suitablegeometry across which a nanotube can be synthesized. Where athrough-hole is preferred, such can be provided in a substrate, as inFIGS. 1C-1D, or through a self-supported membrane or other suitablestructure.

As shown in FIG. 1E, nanotubes 10 can alternatively be synthesizedvertically, grown down or up like blades of grass from the surface of asubstrate 18 on which is provided catalyst regions or a blanket catalystlayer. This synthesis arrangement provides full circumferential accessto the nanotubes for forming a functionalization layer fully around thenanotubes. In this arrangement, the nanotubes are anchored to asubstrate only at one end, and can be maintained in this rootedconfiguration during functionalization and coating processes. Such acondition can also be achieved in a horizontal arrangement, shown inFIG. 1F. Here a nanotube 10 is synthesized from the end of a horizontalstructure, e.g., a beam 22 provided on a substrate 18. A nanotubecatalyst region can be provided at a selected location on the beam oralong the beam length.

In the examples of FIGS. 1A and 1C-1F, one or more carbon nanotubes aresynthesized in place in a selected arrangement that is convenient forcoating the full circumference of the nanotube surface along thenanotube length. This can be extended to synthesis of a nanotube inplace in a selected device or system configuration for which thenanotube is intended, where the full circumference of the nanotube is tobe exposed during operation. Examples of such devices are described indetail below. But as explained above, where such is not the case,nanotubes can be synthesized in any desired fashion and then arranged toexpose their full coaxial surface for functionalization of the fullsurface, or can be manipulated such that repeated functionalizationsteps produce a functionalization layer on the full surface. Where it isnot required to functionalize the full surface of a nanotube, thenanotube can be provided in an arrangement such as FIG. 1B with aselected surface region to be functionalized made accessible to thefunctionalization process.

The invention does not require any particular nanotube synthesis processand is not limited to a particular process. Nanotubes can be obtainedfrom commercial sources or synthesized in a desired manner and withselected characteristics such as wall number and electricalconductivity. Example nanotube synthesis techniques and configurationsthat can be employed in accordance with the invention are described inU.S. Patent Application Publication No. US 2006/0006377, entitled“Suspended Carbon Nanotube Field Effect Transistor,” Published Jan. 12,2006, the entirety of which is hereby incorporated by reference.

For example, nanotube catalyst materials of, e.g., Fe, Co, Ni, alloys ofsuch, or other metals or suitable materials, can be provided in apatterned configuration or a blanket arrangement on a substrate or otherstructure, in the conventional manner, for synthesis of nanotubestherefrom. Where it is desirable to synthesize a nanotube in situ in aselected device configuration, catalyst regions can be provided on,e.g., metal contact pads, in the manner described below, such that aresulting nanotube device configuration includes electrical connectionpoints at ends of the nanotube. For applications in which a nanotube isto be synthesized across a through-hole, the catalyst material can beprovided on a substrate or membrane in an arrangement such that thethrough-hole can be formed in one step through the catalyst and theunderlying substrate or membrane to provide alignment of the edge of thecatalyst region with the edge of the through-hole. For applications inwhich SWNTs are desired, a relatively thin catalyst layer, e.g., of 2 nmin thickness or less, can be preferred.

Nanotube synthesis can be carried out by CVD or other selected vapordeposition process. In one example process, CH₄ is flowed, at a rate of,e.g., about 200 sccm, at a temperature of, e.g., about 900° C., for aselected duration, e.g., up to about 5 minutes in a CVD chamber. In afurther example synthesis process, argon gas is flowed at, e.g., about450 sccm, at atmospheric pressure, through anhydrous liquid ethanol at25° C., room temperature, with the vapor mixture then flowed over acatalyst-coated region at a temperature of, e.g., between about 750°C.-900° C., in, e.g., an Inconel annealing tube, for a duration of,e.g., between about 30 s and 30 min. Other synthesis processes can alsobe employed.

With these example nanotube synthesis techniques, nanotubes can be grownacross through-holes or trenches provided in substrates or membranes,can be grown vertically up or down from a catalyst surface, or can begrown from the edge of a beam or other structure. Multiple nanotubes canbe expected to grow from each catalyst region and remain at leastpartially suspended over holes or trenches or from anchor points such asbeams or other surfaces. Once one or more nanotubes are synthesized orarranged in a manner that accommodates formation of a functionalizationlayer on a selected region of the nanotube or the complete wall surfaceof the nanotube, the functionalization process of the invention iscarried out to enable deposition of additional selected material layerson the nanotube.

The functionalization process of the invention produces at the nanotubesurface a functionalization layer that is substantially conformal anduniform and that provides chemically functional groups amenable todeposition of a material layer thereon and/or that can react withcoating precursors such as ALD or CVD precursor molecules. Thefunctionalization layer does not, however, significantly disrupt theelectronic structure or other properties of the nanotube and thereforeis benign to the nanotube. Accordingly, in the functionalizationprocess, there is delivered to the nanotube surface a species that canphysically attach to the surface for uniformly coating the surface butwhich does not chemically react with the surface. The functionalizationlayer is physisorbed at the nanotube surface but is not chemisorbed atthe nanotube surface.

Such a condition can for many cases be characterized as non-covalentbonding. Covalent bonding typically characterizes a condition in whichlocalized interaction between species can trap electrons and disruptelectronic properties. The functionalization process of the inventionavoids this condition by employing a functionalization species that doesnot result in localized electronic interaction with the nanotubesurface. The non-covalent physical bond of the functionalization speciesto a nanotube surface is strong enough to hold the species in place forat least a limited duration, but is reversible, and therefore can desorbfrom the surface. With this condition, the physical bond is not of anature that perturbs the structure of the nanotube surface.

One example functionalization species that can be employed in accordancewith the invention is NO₂. NO₂ is characterized by a high bindingefficiency that results from low lying electronic states. This highbinding efficiency, in combination with a tendency to strong Van derWaals attraction, enables NO₂ to physically bond to a nanotube surfacewithout chemically bonding to the surface. Without chemical bonding tothe surface, the NO₂ does not strongly perturb the surfaceelectronically or chemically. NO₂ can be delivered to a nanotube surfaceas a gaseous vapor species. For many applications, it is preferred inaccordance with the invention that the functionalization species be avapor species, rather than a liquid species, for enabling precision indeposition as well as for convenience. NO₂ thereby is particularlywell-suited as a functionalization species.

The characteristics of NO₂ given just above generally describe therequirements of a functionalization species of the invention. Thefunctionalization species should be adsorbed on and non-covalentlyphysically bond to, or physisorbed on, a nanotube surface withoutsignificantly perturbing the chemical and/or electronic states of thenanotube. The physisorbed functionalization species should be capable ofoperating as nucleation sites for reaction with one or more precursorsfor depositing a material layer on the nanotube. The functionalizationspecies should also preferably be deliverable to a nanotube surface inthe vapor phase.

The invention is not limited to the NO₂ functionalization example. Anyspecies that provides the surface-bonding and precursor-reactioncharacteristics described above can be employed as a functionalizationspecies. For example, methyl nitrite gas, CH₃ONO, can be employed inplace of NO₂ gas.

Species and processes that result in covalent bonding with a nanotubesurface cannot be employed in the functionalization process of theinvention. For example, layers of some metals, such as Ti and Cr, areunderstood to impact the electronic state of nanotubes and to covalentlybond to the nanotube surface. As a result, such metals cannot beemployed as a functionalization species in the functionalization processof the invention. In contrast, NO₂ does not substantially alter theelectrical state of nanotubes to which it is attached. Any nanotubeconduction enhancement produced by an NO₂ coating, e.g., a slight p-typeconduction enhancement that is known to be characteristic of NO₂ coatingon nanotubes, is almost entirely due to electrostatic modification atthe metal/nanotube interface by Schottky barrier modification, asdescribed in detail below. NO₂ does not interact electronically in thesense that covalent bonds are not formed with the nanotube lattice andthe hybridization state, i.e., the electronic state, of the nanotube istherefore not changed by the NO₂.

Furthermore, as metals, Cr and Ti, as well as other electricallyconducting materials, cannot be employed as a surface coating on ananotube that is to be employed as, e.g., a semiconducting element suchas a transistor gate. A metal layer on a nanotube gate surface woulddominate electrical current transport along the nanotube gate byproviding a pathway for electrons to travel through the metal, ratherthan the semiconducting nanotube, thereby eliminating the requisitetransistor gate switching capabilities.

Particular deposition processes can also alter the properties of ananotube and are therefore cannot be employed as a functionalizationprocess. For example, some high-temperature CVD processes, typicallyoperated at temperatures over 400° C., are known to deleteriously impactthe electrical transport properties of nanotubes by changing theelectronic structure of nanotubes. This is understood to occur due to abonding arrangement that is not non-covalent. In other words, thehigh-temperature CVD process causes formation of covalent bonds with ananotube that change the electrical properties of the nanotube. Incontrast, the low-temperature vapor deposition process of the inventiondoes not form with a nanotube covalent bonds that could alter theelectrical properties of the nanotube.

Considering now the NO₂ functionalization species in more detail, it isfound that the physical bond of NO₂ species to a nanotube surface isreversible, as mentioned above. In other words, adsorbed NO₂ moleculestend to desorb from a nanotube surface over time. As a result, althougha layer of NO₂ molecules coated on a nanotube surface will act asnucleation sites for deposition of a selected material layer on thenanotube, the NO₂ molecules will not in general remain on the nanotubesurface for a duration that is sufficiently long to enable the desiredmaterial layer deposition. The challenge is to form a continuous layerof NO₂ that is preserved for a duration sufficient to carry out aselected material deposition process.

In accordance with the invention, this is achieved by depositing,preferably from the vapor phase, a layer of NO₂ at a temperature thatpreserves NO₂ molecules on a nanotube surface for a duration sufficientto introduce a second functionalization species to react with the NO₂molecules and form a species that is not so easily desorbed from thenanotube surface. The reaction of the NO₂ and the secondfunctionalization species forms a more stable complex that does not sorapidly desorb from a nanotube surface and that maintains nucleationsites for deposition of a material and/or reaction with a selecteddeposition precursor.

Any suitable species that is sufficiently volatile and reactive to bindto the NO₂ and stabilize the NO₂ on the surface of a nanotube can beemployed in the functionalization technique. One example and preferredreactive second functionalization species is Al(CH₃)₃ (trimethylaluminumor TMA), but other reactants, such as dimethylzinc, trimethylgallium,trimethylindium, trimethylbismuth, tetrakis(dimethylamido)hafnium ortetrakis(dimethylamido)zirconium, can also be used. Whatever reactivesecond functionalization species is selected, it preferably binds to theNO₂ and stabilizes the NO₂ on the surface of a nanotube. The inventionis not limited to a particular reactive second functionalizationspecies, and instead requires only that the result of a reaction of theselected species with, e.g., NO₂ or other selected firstfunctionalization species, not strongly perturb the electronic orchemical state of the nanotube and provide nucleation sites fordeposition of a material layer on the nanotube surface.

In one example and well-suited process provided by the invention, acyclic ALD technique can be employed to introduce vapor NO₂ to ananotube surface and then to expose the adsorbed NO₂ molecules to vaporAl(CH₃)₃, such that the Al(CH₃)₃ reacts with the NO₂ molecules. Thereaction of the Al(CH₃)₃ with the NO₂ produces a functionalization layerthat is non-covalently bonded to the underlying carbon nanotube and thatis a stable complex which does not tend to immediately desorb from thenanotube sidewall at a reasonably low temperature, such as roomtemperature, 25° C. The resulting functionalized surface providesfunctional groups that are amenable to uniform, conformal coating of thenanotube by a selected additional material or materials.

A plausible reaction mechanism for adsorbed NO₂ with TMA isschematically illustrated in FIG. 2. Here, the nitrogen end of NO₂ isattracted to the nanotube surface, leaving the oxygen ends exposed toincoming TMA vapor. This is the most stable configuration for adsorbedNO₂ molecules on the nanotube surface. The aluminum centers of TMA arein turn attracted to oxygen, leaving the methyl groups as the surfacefunctional groups that are available for reaction with depositionprecursors and amenable to deposition conditions. As explained below,this configuration results in self-terminating behavior under a range orprocessing conditions, preventing further attachment of either NO₂ orTMA.

In one example NO₂-TMA functionalization process in accordance with theinvention, a cyclic ALD technique is employed at a temperature thatgenerally minimizes a tendency of molecular desorption, e.g., about roomtemperature, 25° C. Throughout the entire functionalization process, aselected flow, e.g., about 50 sccm, of a selected inert gas, e.g., argongas, is directed through an ALD reactor. An example of such is acylindrical reactor having an inner diameter of 3.4 cm. The inert gasflow is controlled, e.g., by a vacuum pump with a pumping speed suchthat the chamber pressure is maintained at, e.g., about 300 mTorr. Oneexample NO₂-TMA functionalization cycle consists of a first cycle stepproviding a dose of, e.g., about 30 mL, of NO₂ gas at a pressure of,e.g., about 960 Torr. After this NO₂ dose there is carried out an NO₂purge, e.g., a 7 s purge with argon. Then a second cycle step is carriedout with a dose of, e.g., about 6 mL, of TMA vapor at a pressure of,e.g., about 10 Torr. This TMA dose is then followed by a 2 min purgewith argon. This final purge ends a NO₂-TMA cycle. As explained below,the number of such cycles to be repeated is determined based on thecompleteness of coverage provided by a selected functionalization layer.As will be recognized, the dose size, pressure, and other processparameters are dependent on reactor size and therefore are to beadjusted accordingly for a given reactor and other process conditions.

With this example ALD functionalization process, the residence time, orlife time, of the adsorbed NO₂ species resulting from the 30 mL dose ofNO₂ gas is sufficient for the adsorbed NO₂ species to remain on thenanotube surface until the TMA vapor is introduced for reaction with theadsorbed NO₂ species. The example process temperature of roomtemperature is one process parameter that can be easily controlled toimpose this condition. For many applications, a process temperature ofabout room temperature, 25° C., can be preferred, but the invention isnot limited to room temperature processing. The processing temperatureis preferably less than about 200° C. and can be lower than roomtemperature if such is suitable for a given application.

For many applications, a functionalization layer of one monolayer or atmost a few monolayers is all that is required to provide the functionalgroups necessary for nucleation of a selected material to be depositedsubsequently on the functionalization layer surrounding a nanotube.Relatively thick functionalization layers may be porous and notoptimally uniform or conformal. In addition, for many applications itcan be preferred to limit the amount of material applied to a nanotube.It can therefore be preferred to limit the thickness of afunctionalization layer, either through active control or by selectionof a process that is characterized by self-limiting behavior.

The example NO₂-TMA functionalization process described above ischaracterized by such self-limiting behavior, limiting a resultingfunctionalization layer thickness to one monolayer, which is less thanabout 1 nm in thickness. The self-limiting nature of this reaction canbe preserved by limiting the number of functionalization ALD cycles toless than 100 cycles, so that only a controlled, self-limited monolayerof material forms. If the dose number is too large or exposure toogreat, the NO₂-TMA process can produce a thicker amorphous multilayerthat is not self-limiting, as described in detail below. Such amultilayer is not a true ALD process and cannot be guaranteed to providethe conformality and uniformity required by the invention. For manyapplications, less than 100 cycles can be preferred, or more preferably,less than 50 cycles. The number of cycles to be employed should besufficient to fully or at least nearly completely cover a nanotubesurface. For most applications, preferably more than 10 cycles, and morepreferably, more than 25 cycles are sufficient for nearly completecoverage.

In general, once a nanotube surface has been exposed to the exampleNO₂-TMA ALD process described above or other selected process, afunctionalization layer exists on the nanotube surface and deposition ofa selected material around the nanotube can proceed. But although theadsorbed NO₂-TMA complex is more stable than adsorbed NO₂ by itself,because the NO₂-TMA complex is non-covalently attached to the nanotube,the complex will tend to desorb from the nanotube surface given asufficient amount of time and a sufficiently high temperature.

For conditions under which a selected material cannot be deposited priorto the onset of such functionalization layer desorption, an intermediatestabilization layer can be applied in accordance with the invention toencase the functionalization layer and ensure stability of thefunctionalization layer even at elevated temperatures and extendeddurations. The stabilized functionalization layer providesfunctionalized nucleation sites such that deposition of a selectedmaterial layer can then be carried out on the nanotube. Following thefunctionalization and stabilization steps, the structure can be heatedto higher temperatures at which the initial functionalization layer mayhave desorbed without the stabilization process.

The stabilization process preferably is conducted at a temperature thatdoes not cause enhanced desorption of the underlying functionalizationlayer. For many applications, a temperature around room temperature, 25°C., can be a preferable stabilization process temperature; a temperaturebelow about 200° C. can be preferred, with temperatures below about 100°C. more preferable. The stabilization layer preferably just covers theunderlying functionalization layer and therefore need be no more thanone or just a few monolayers in thickness. The stabilization layer holdsthe functionalization layer in place and preferably forms strongchemical bonds, such as covalent bonds, with the functionalizationlayer, although such strong chemical bonds are not essential to theinvention.

In one example technique, a selected precursor is introduced to afunctionalization layer-coated nanotube for reaction with the layer. Thecriteria to be met include a requirement that the deposition precursoror precursors for the stabilization layer be reactive with the speciesof the functionalization layer, e.g., with NO₂-TMA functionalizationlayer species, and be sufficiently volatile at the temperature requiredto maintain the functionalization layer, e.g., about 25° C., forsuccessful delivery of their vapors to the functionalization layersurface. Precursors that meet these criteria include dimethylamides ofvarious metals, such as hafnium and zirconium, and methyl derivatives ofother metals, such as aluminum, zinc, gallium and bismuth. Reaction ofsuch precursors, including trimethylaluminum, dimethylzinc,trimethylgallium, trimethylindium, trimethylbismuth,tetrakis(dimethylamido)hafnium or tetrakis(dimethylamido)zirconium withwater vapor can be carried out to form a stabilization layer. It isunderstood that a NO₂-TMA functionalization layer can be exposed to atemperature around 50° C. and for most conditions still be characterizedby a reasonably small desorption rate; at this higher temperature anexpanded class of stabilization layer precursors can be employed.

In one example and convenient process, a layer of Al₂O₃ can be depositedby ALD to form a monolayer-thick stabilization layer over afunctionalization layer. Where the functionalization layer is formed byan ALD process, an ALD stabilization process can be particularlyconvenient. In an example of such a process, a selected flow, e.g.,about 50 sccm, of a selected inert gas, e.g., argon gas, is directedthrough an ALD reactor. An example of such is a cylindrical reactorhaving an inner diameter of 3.4 cm. The inert gas flow is controlled,e.g., by a vacuum pump with a pumping speed such that the chamberpressure is maintained at, e.g., about 300 mTorr. The temperature ismaintained at about room temperature, 25° C. The first step in onestabilization cycle consists of a dose of, e.g., about 6 mL of TMA, at apressure of, e.g., about 10 Torr, followed by a 2 min purge with argon.Then in a second process step, water vapor at a pressure of, e.g., about24 Torr, is pulsed for 0.2 sec. The chamber is then purged with argonfor, e.g., about 2 min. This completes one ALD cycle in the formation ofa stabilization layer.

It is found that for an NO₂-TMA functionalization layer, five Al₂O₃ ALDcycles with the process conditions given just above provide an adequatestabilization layer, and thus a process including at least five cyclescan be preferred. The thickness of an Al₂O₃ stabilization layer producedby the five ALD cycles is found to be sufficient to keep NO₂-TMAcomplexes held in place on a nanotube for high temperature processing,and yet is thin enough to minimize any adverse effects that thestabilization layer could have on electrical capacitance.

Thus, in accordance with the invention, in one example of a particularlypreferable process, 25-50 ALD cycles of functionalization by NO₂-TMA,followed by at least 5 ALD cycles of stabilization by Al₂O₃, all atabout 25° C., sufficiently functionalizes and stabilizes a nanotubesurface for deposition by a selected process, e.g., by conventional ALD.Most nanotubes, and in particular SWNTs, treated with this process ofthe invention exhibit an ideal coating that is uniform and smooth,conformal and continuous along the entire length of the nanotube.

It is found that without a stabilization layer, a functionalizationlayer can be radially isotropic but may not in all cases remaincontinuous along the nanotube length, e.g., producing uncoated regionsdue to desorption. A similar undesirable condition is found to exist ifwater vapor alone is employed as a stabilization precursor in an effortto cross-link NO₂-TMA complexes in an underlying function layer. Anonuniform functionalization layer is also found to result if thefunctionalization procedure is carried out at conventional ALDtemperatures, which are generally 200° C.-400° C. Here, the thermalkinetic energy gained by NO₂ molecules causes a decrease in adsorptiononto a nanotube surface, inhibiting functionalization. It is thereforepreferred in accordance with the invention that low temperature ALDtechniques be employed for the functionalization and stabilizationprocesses.

Once the functionalization and stabilization steps of the invention havebeen completed, the carbon nanotube can be coated with a selectedmaterial by any of a wide range of physical, chemical, vapor, andsolution-based deposition processes. Where the deposition process isitself a vapor process, e.g., a CVD process or an ALD process, SWNTs orMWNTs can be grown, their surfaces functionalized and stabilized inaccordance with the invention, and their surfaces coated with ALD or CVDmaterial, all in the same reactor. The nanotubes can therefore be keptin a controlled vacuum environment throughout the entire processsequence. This eliminates all sources of contamination and ensures thatthe selected precursors are indeed directly responsible for each step.The ability to execute every process step in one reactor alsoillustrates the ease with which the functionalization technique of theinvention can be integrated into a fabrication process sequence.

It is understood that the conditions for producing a selected layer ofstabilization material may not be ideal for producing a layer of thatsame material as-intended for another purpose. But for manyapplications, the stabilization layer can be formed of a material thatis also to be subsequently deposited in the formation of a selecteddevice. It can therefore be preferred for such a scenario to first forma stabilization layer of the minimum thickness required for stabilizingthe functionalization layer, and then to adjust the depositionconditions to form an intended deposited material. For example, asdescribed in detail below, where Al₂O₃ is to be employed as astabilization material and as a device layer in an intended deviceconfiguration, then a low-temperature Al₂O₃ deposition step can beconducted, followed by an Al₂O₃ deposition at temperature and pressuresettings that optimize the Al₂O₃ layer for the intended application.

The invention contemplates a wide range of devices and structures thatcan be produced with carbon nanotubes having surfaces that arefunctionalized in accordance with the invention. A wide range ofmaterials can here be employed. For example, a functionalized carbonnanotube can be coaxially coated with a selected dielectric layer andthen a selected electrically conducting layer, to form a gate structurethat can be configured between source and drain regions in the formationof a carbon nanotube-based transistor.

One particularly important application addressed by the methods of theinvention is ALD of a high-x dielectric coating, having a dielectricconstant greater than, e.g., about 7, on a SWNT. Formation of a high-κdielectric layer by ALD can produce a benign dielectric/SWNT interfacethat does not adversely affect the electrical properties of thenanotube. ALD is thus an ideal deposition technique for nondestructivelypassivating SWNTs with very thin, high-κ materials. Coaxial coating isparticularly desirable because its realization in accordance with theinvention makes coaxially-gated SWNT devices and nanotube-based wiresand other circuit elements feasible, as described below. Thisconfiguration maximizes the capacitive coupling of the nanotube with agate layer, which is a crucial step towards optimizing devicecharacteristics. Furthermore, a suspended SWNT geometry may be necessaryto satisfy scalability requirements for nanoscale SWNT devices. Such astructure necessitates coaxial insulation.

It is discovered in accordance with the invention that for such acoaxially-insulated and coaxially-gated SWNT configuration, thefunctionalization and stabilization layers can under certain conditionseffect the capacitive response of such a configuration to appliedvoltages. Beyond SWNT devices, for many nanoscale microelectronicapplications, it can be preferable to conduct electrical analysis of astructure including the functionalization and stabilization layers todetermine if non-ideal capacitive properties are produced by thefunctionalization layer. This can be especially preferred for electronicapplications, but is not in general necessary for other non-electronicapplications.

In accordance with the invention, if required, there can be carried outa post-functionalization anneal process in accordance with the inventionto reduce or eliminate non-ideal capacitive properties of afunctionalized structure. The annealing process can be conducted in aconventional furnace that is thermally ramped, in a rapid thermal anneal(RTA) chamber, or other suitable heating system. The anneal process ispreferably carried out at a temperature that is between about 300° C.and about 700° C. For many applications, an anneal temperature that isbetween about 500° C. and about 600° C. can be preferred tosubstantially completely eliminate unwanted capacitive characteristics.At anneal temperatures above about 700° C., degradation of capacitivecharacteristics can occur. The optimum anneal temperature can bespecific to a selected structure geometry, material composition, andfunctionalization configuration, depending on the properties of thestructures and the nature of the functionalization of the structures.

In accordance with the invention, the anneal temperature, time, and rampspeed are selected specifically to remove energy levels that exist inthe functionalization and stabilization layers and that function aselectrical charge traps. These electrical charge traps can degradeelectronic device performance. Therefore, depending on the selectedanneal system and anneal temperature, the duration of the anneal ispreferably that duration which is sufficient to reduce the charge trapsof the functionalization and stabilization layers to an acceptable levelfor a given application. An anneal time is therefore set preferablybased on the particular annealing technique employed, and can rangefrom, e.g., between about 1 minute and about 5 hours, and morepreferably between about 2 minutes and about 2 hours. It is to berecognized that given a particular selected anneal system and annealtemperature, some empirical analysis can be required to ascertain anoptimal anneal duration.

Preferably, the anneal process is not carried out in a reactive gasenvironment and there is no function for a reactive gas in the annealprocess. The anneal therefore can be conducted in vacuum or an inertgas, such as Ar, that is compatible with the materials present on thestructure to be annealed. The anneal process of the invention can beconducted immediately following formation of functionalization andstabilization layers, or can be carried out at a later point in afabrication process sequence, after additional material layers aredeposited on a structure. For many applications, it can be preferred toconduct the anneal prior to metal deposition on a functionalizedstructure and to minimize the anneal temperature to preserve the thermalbudget of a fabrication sequence.

The surface functionalization processes and the annealing processes ofthe invention can be applied to any structure having carbon surfaces,and therefore are not limited to single-walled carbon nanotubeprocessing. Any structure having carbon surfaces, e.g., bulk graphite,such as graphite substrates or chips, graphene sheets or layers providedon a substrate or other structure, buckyballs, or other fullerenestructures that exhibit inertness to ALD or CVD precursors or otherdeposition species, in the manner of carbon nanotubes, can befunctionalized by the processes of the invention to overcome suchinertness. Once functionalized, ALD or other deposition or growthtechnique as described above can be employed to form one or more layersof materials on the carbon surface. These layers of materials can beprovided for, e.g., passivation, electrical isolation, voltage biasing,chemical, mechanical, or environmental shielding, or other functionrelated to an intended application for the structure.

For example, conventional precursors for ALD and CVD processes do not ingeneral nucleate easily on the flat surfaces of graphite crystallites.Instead, nucleation takes place only at the edges of crystallites and atother defect sites, from which a coating can grow and eventually extendto cover completely the graphite. But if it is desired to completelycover a graphite surface with a thinner and more uniform layer ofmaterial, then the functionalization and stabilization processes of thepresent invention can be employed to form very thin functionalizationand stabilization layers, onto which additional material can be added,for example by ALD and/or CVD.

Graphite is a planar carbon-based structure in the family ofcarbon-based structures that also includes the planar structure grapheneand non-planar fullerenes such as carbon nanotubes that are processed inaccordance with the invention with functionalization and stabilizationlayers to enable deposition of material layers on the carbon structuresurface. The term “fullerene structure” is herein meant to refer to anymaterial composed mainly of ordered hexagonal groups of carbon atoms orhexagonal and pentagonal groups of carbon atoms that is shaped in anon-planar geometry. Graphene is an example of a planar, single layer ofhexagonal carbon atom groups. Bulk graphite is a planar structure formedof multiple layers of graphene. Non-planar fullerene structurearrangements and layers of carbon-based materials can also be subjectedto the functionalization, stabilization, and anneal processes of theinvention. In general, any three-dimensional fullerene or carbon-basedstructure, whether planar, coaxial, spherical, as in a buckeyball (C₆₀)configuration, or other configuration, can be functionalized,stabilized, and annealed in accordance with the invention to enableformation of material layers on the fullerene surface. Thus, bulkgraphite, having a surface that is chemically similar to that of acarbon nanotube fullerene structure, can be processed in a similarmanner in accordance with the invention.

In general, a carbon-based structure to be functionalized is firstprepared in a geometry that is amenable for functionalization and anintended application. Graphite substrates or graphite sheets can beobtained commercially and provided in a suitable configuration.Fullerene structures can be synthesized in situ as described above orprovided at an intended location after synthesis.

Graphene can be provided on a substrate or other structure by anysuitable method. In one conventional technique, in a first step,commercially-available highly-oriented pyrolytic graphite or graphiteflakes are attached to plastic sticky tape, with the tape folded overthe graphite. The tape is then peeled apart to cleave or exfoliate thegraphite. This process is repeated a number of times, e.g., 10 times, onthe increasingly thinner material. Bulk graphite or graphite flakes canalso first be suitably attached to, e.g., wet photoresist, provided on asubstrate; after baking the resist, the graphite is attached to theresist and using sticky plastic tape, flakes of graphite can berepeatedly peeled from the bulk.

Once flakes are produced, the flakes are applied to an intendedsubstrate, e.g., an oxidized silicon wafer. Graphitic films that arethinner than about 50 nm are transparent to visible light butnevertheless can easily be seen on an oxide surface through a microscopebecause of the added optical path that shifts the interference colors.At a thickness of less than about 1.5 nm, corresponding to the thicknessof graphene, no color is visible even via the interference shift. Thisprovides a natural marker for distinguishing between multi-layergraphite and graphene; regions of graphene appear as highly-transparentcrystalline shapes having little color compared with the rest of theoxide layer.

Graphene multilayers can also be grown epitaxially on single crystalsilicon carbide substrates. For example, in ultra-high vacuum, 4H- and6H—SiC crystals can be heated to a temperature of about 1300° C. tocause graphene or graphitic films to grow on both the silicon-terminatedand carbon-terminated faces of the crystals. More moderate vacuumconditions for the growth can be employed with controlled backgroundgas. Growth on the Si face is slow and terminates after relatively shorttimes at high temperatures, but growth on the carbon face does notself-limit such that a number of ranging between 1-100 can be produced.The layers can then be employed as-grown or can be exfoliatedas-described above to produce graphite layers or graphene. The inventionis not limited to a particular technique for producing graphite orgraphene and contemplates the use of advanced techniques in thesynthesis of graphene that are not now available.

With a selected graphite, graphene, or fullerene structure synthesizedand suitably configured at a selected structure, the functionalizationand stabilization processes of the invention are carried out to providefunctional groups, and specifically —CH₃ methyl groups, at the carbonsurface for enabling nucleation by deposition species such as ALDprecursors.

In one example NO₂-TMA functionalization process in accordance with theinvention, the cyclic ALD technique described above is employed at atemperature that generally minimizes a tendency of molecular desorptionfrom the carbon-based surface, e.g., about room temperature, 25° C.Throughout the entire functionalization process, a selected flow, e.g.,about 50 sccm, of a selected inert gas, e.g., argon gas, is directedthrough an ALD reactor. An example of such is a cylindrical reactorhaving an inner diameter of 3.4 cm. The inert gas flow is controlled,e.g., by a vacuum pump with a pumping speed such that the chamberpressure is maintained at, e.g., about 300 mTorr. One example NO₂-TMAfunctionalization cycle consists of a first cycle step providing a doseof, e.g., about 30 mL, of NO₂ gas at a pressure of, e.g., about 960Torr. After this NO₂ dose there is carried out an NO₂ purge, e.g., a 7 spurge with argon. Then a second cycle step is carried out with a doseof, e.g., about 6 mL, of TMA vapor at a pressure of, e.g., about 10Torr. This TMA dose is then followed by a 2 min purge with argon. Thisfinal purge ends a NO₂-TMA cycle.

The reaction mechanism for adsorbed NO₂ with TMA at a graphite,graphene, or fullerene structure surface is the same as that shown for acarbon nanotube surface in FIG. 2. Here, the nitrogen end of NO₂ isattracted to the graphene, graphite, or other planar or fullerenesurface, leaving the oxygen ends exposed to incoming TMA vapor. This isthe most stable configuration for adsorbed NO₂ molecules on the carbonsurface. The aluminum centers of TMA are in turn attracted to oxygen,leaving the methyl groups as the surface functional groups that areavailable for reaction with deposition precursors and amenable todeposition conditions. This configuration results in self-terminatingbehavior described above, preventing further attachment of either NO₂ orTMA.

With the functionalization layer so-formed on a carbon surface, thestabilization layer is then formed on the functionalization layer in themanner described above. In one example stabilization technique, aselected precursor is introduced to a functionalization layer-coatedcarbon-based structure for reaction with the layer. The criteria to bemet include a requirement that the deposition precursor or precursorsfor the stabilization layer be reactive with the species of thefunctionalization layer, e.g., with NO₂-TMA functionalization layerspecies, and be sufficiently volatile at the temperature required tomaintain the functionalization layer, e.g., about 25° C., for successfuldelivery of their vapors to the functionalization layer surface.Precursors that meet these criteria include dimethylamides of variousmetals, such as hafnium and zirconium, and methyl derivatives of othermetals, such as aluminum, zinc, gallium and bismuth. Reaction of suchprecursors, including trimethylaluminum, dimethylzinc, trimethylgallium,trimethylindium, trimethylbismuth, tetrakis(dimethylamido)hafnium ortetrakis(dimethylamido)zirconium with water vapor can be carried out toform a stabilization layer. It is understood that a NO₂-TMAfunctionalization layer can be exposed to a temperature around 50° C.and for most conditions still be characterized by a reasonably smalldesorption rate; at this higher temperature an expanded class ofstabilization layer precursors can be employed.

In one example and convenient process, a layer of Al₂O₃ can be depositedby ALD to form a monolayer-thick stabilization layer over afunctionalization layer. Where the functionalization layer is formed byan ALD process, an ALD stabilization process can be particularlyconvenient. In an example of such a process, a selected flow, e.g.,about 50 sccm, of a selected inert gas, e.g., argon gas, is directedthrough an ALD reactor. An example of such is a cylindrical reactorhaving an inner diameter of 3.4 cm. The inert gas flow is controlled,e.g., by a vacuum pump with a pumping speed such that the chamberpressure is maintained at, e.g., about 300 mTorr. The temperature ismaintained at about room temperature, 25° C. The first step in onestabilization cycle consists of a dose of, e.g., about 6 mL of TMA, at apressure of, e.g., about 10 Torr, followed by a 2 min purge with argon.Then in a second process step, water vapor at a pressure of, e.g., about24 Torr, is pulsed for 0.2 sec. The chamber is then purged with argonfor, e.g., about 2 min. This completes one ALD cycle in the formation ofa stabilization layer. Thus, in accordance with the invention, in oneexample of a particularly preferable process, 25-50 ALD cycles offunctionalization by NO₂-TMA, followed by at least 5 ALD cycles ofstabilization by Al₂O₃, all at about 25° C., sufficiently functionalizesand stabilizes a carbon-based structure surface such as a graphitesurface or a graphene surface, for deposition by a selected process,e.g., by conventional ALD, of materials on the graphite or graphene.

For a given microelectronic application of a functionalized graphite,graphene structure or fullerene structure, the annealing processes ofthe invention can be employed, if necessary, to reduce or eliminatenon-ideal capacitive characteristics of the structure that result fromthe functionalization. The annealing process conditions given above canbe employed here, with an inert gas flowing during the annealingprocess. The anneal process is preferably carried out at a temperaturethat is between about 300° C. and about 700° C. For many applications,an anneal temperature that is between about 500° C. and about 600° C.can be preferred to substantially completely eliminate unwantedcapacitive characteristics. An anneal time is set preferably based onthe particular annealing technique employed, and can range from, e.g.,between about 1 minute and about 5 hours, and more preferably betweenabout 2 minutes and about 2 hours. It is to be recognized that theanneal temperature and duration can differ between various carbon-basedmaterials, structures, and configurations such as fullerene structureslike carbon nanotubes, and planar or other three-dimensionalconfigurations,

The inventors herein have made the discovery that functionalization andstabilization layers produced in accordance with the invention canprovide support for coatings and layers on structures that are notcarbon-based materials, including, e.g., silicon, silicon dioxide,silicon nitride, glasses, and metals, and other microelectronicmaterials that are chemically different from the carbon-based family ofgraphite, graphene, and fullerene structures described above. Thefunctionalization and stabilization layers of the invention can beformed on insulating, semiconducting, or conducting structures. Themicroelectronic structures can be planar, and further can be coaxial,spherical, or other selected configuration. Microelectronic wafers andsubstrates or pieces of substrates, chips, or other structures can beprocessed with the functionalization and stabilization processes of theinvention.

In general, any structure for which a surface to accommodate materialdeposition is needed can be functionalized, if the surface of thestructure is compatible with the functionalization chemistry. Asexplained above, in one of a particularly preferable process, 25-50 ALDcycles of functionalization by NO₂-TMA, followed by at least 5 ALDcycles of stabilization by Al₂O₃, all at about 25° C., sufficientlyfunctionalizes and stabilizes a structure surface such as a siliconwafer or other structure, for deposition by a selected process, e.g., byconventional ALD, of materials on the structure.

For a given microelectronic application of a functionalizedmicroelectronic wafer or other structure, the annealing processes of theinvention described can be employed, if necessary, to reduce oreliminate non-ideal capacitive characteristics of the structure thatresult from the functionalization. The anneal process is preferablycarried out in with flow of an inert gas, at a temperature that isbetween about 300° C. and about 700° C. For many applications, an annealtemperature that is between about 500° C. and about 600° C. can bepreferred to substantially completely eliminate unwanted capacitivecharacteristics. An anneal time is set preferably based on theparticular annealing technique employed, and can range from, e.g.,between about 1 minute and about 5 hours, and more preferably betweenabout 2 minutes and about 2 hours.

EXAMPLE I NO₂ Adsorption and Desorption from a Carbon Nanotube

The electrical conductance of SWNTs was experimentally analyzed duringthe functionalization procedure of the invention. To produce structuresfor analysis, optical lithography and electron beam evaporationtechniques were employed to pattern metal electrodes of 50 nm-thick Moand 50 nm-thick Ti layers on quartz substrates. A focused ion beam (FIB)was employed to form trenches of 1 μm in width and 0.5 μm in depthbetween the edges of the metal electrodes. A layer of approximately 0.5nm in thickness of Al₂O₃ was deposited by ALD on the electrode surfacesto prevent alloying between the molybdenum and the nanotube catalystmetal subsequently deposited. A catalyst region of Co of about 0.3 nm inthickness was deposited and patterned by liftoff on the electrodesurfaces to form isolated catalyst regions.

SWNT synthesis was carried out at atmospheric pressure by bubbling argonat a flow rate of about 450 sccm through ethanol at room temperature,25° C., and flowing the mixture over the quartz substrate structures ata temperature between about 750° C. and about 900° C. in an Inconel®tube having a 3.4 cm inner diameter and a 860 mL volume. Growth timeswere varied between 30 s and 5 min. The longer growth times producedlonger tubes.

For electrical conductance studies, a 30 s growth time was used, toavoid surface bridging of the electrodes by nanotubes. This was notdesirable given that the conductance properties of suspended nanotubeswere being studied. Growing for a relatively short time ensured that thetubes bridging the electrodes were suspended over the trench between theelectrodes. During the synthesis, SWNTs became electrically accessibleby growing over the trench between the electrodes. Semiconducting SWNTsgrown in this way exhibit p-type behavior.

To analyze the adsorption of NO₂ on a nanotube surface, approximately 30mL doses of NO₂ at a pressure of about 960 Torr were pulsed over theSWNTs suspended across the trenches, between the electrodes, understeady state conditions in vacuum conditions of about 300 mTorr and atemperature of about 25° C. Between each pulse, the NO₂ was purged witha continuous flow of argon of about 50 sccm for 2 min.

FIG. 3A is a plot of the measured conductance of one of the suspendedSWNTs as a function of time as the NO₂ pulsed dosing was carried out. Itwas found that the measured conductance of the SWNTs increased upon NO₂exposure and was followed by recovery to the initial baselineconductance during the purge. This is expected because in semiconductingSWNTs, adsorption of the NO₂ slightly enhances the p-type character ofthe nanotube through partial charge transfer of electrons from the tubeto the adsorbed NO₂ molecules. Thus, during the NO₂ dosing, theconductance increased. Then, when the NO₂ was purged, the adsorbed NO₂began to desorb, reducing the p-type character of the nanotube andcorrespondingly reducing the conductance of the nanotube. Thisconductance analysis confirms that the NO₂ species is only physisorbedto the nanotube surface and is not chemically bonded to the surface.

EXAMPLE II TMA Stabilization of Adsorbed NO₂

Suspended SWNTs were fabricated in the configuration of Example I andexposed to pulsed dosing of TMA. Here approximately 6 mL doses of TMA ata pressure of about 10 Torr were pulsed over suspended SWNTs under thesame conditions that were used in the NO₂ pulsing of Example I. Duringthe pulsed dosing the conductance of the SWNTs was measured.

FIG. 3B is a plot of measured conductance of a SWNT as a function oftime during pulsed TMA dosing. TMA exposure has an effect on theconductive properties of SWNTs that is the opposite of that due to NO₂exposure, and thus causes a decrease in SWNT conductance. The measuredSWNT conductance following a TMA pulse exposure is seen in the plot torecover slightly due to desorption after each pulse, but it does notreturn to a constant baseline value as it did with NO₂ exposure. Thisresults in an overall negative slope in the conductance/time profile,which may be due to an increase in contact resistance at the Moelectrodes caused by the reaction of TMA with surface hydroxyls.

The NO₂ dosing experiment of Example I was combined with the TMA pulseddosing, so that each NO₂ pulse was followed by a TMA pulse. Theconductance of a SWNT was measured during the dosing pulse sequence.FIG. 3C is a plot of the measured conductance during the pulsed dosing,and FIG. 3D is a plot of conductance for one pulse cycle of thesequence. Here it is found that each conductance increase caused by NO₂is offset by a conductance decrease caused by the TMA. Recall that whenNO₂ was dosed by itself, the conductance returned to a baseline value,and the average slope was zero. This was not the case when NO₂ dosingwas followed by TMA exposure. Here the current rises stepwise with everyNO₂-TMA cycle, and there is a net positive slope.

If NO₂ could freely desorb from the SWNT surface, one would expect theslope of FIG. 3C to be zero as in the plot of FIG. 3A, or even negativebecause of the presence of TMA. The positive net slope produced by theNO₂-TMA pulsed treatment therefore indicates that TMA reacts with NO₂ toeffectively inhibit desorption of the NO₂, resulting in a more stablecomplex on the SWNT surface.

EXAMPLE III Functionalization Layer Thickness Control by ALD CycleNumber

SWNTs were synthesized as in Example I. The SWNTs were subjected tovarious NO₂-TMA functionalization processes, with the number of NO₂-TMAfunctionalization cycles adjusted to elucidate the nature of theresulting functionalization layer. For each process, a vacuum pressureof about 300 mTorr, under the continuous flow of argon at a flow rate ofabout 50 sccm, at a temperature of about 25° C., was employed. Onefunctionalization cycle consisted of a 30 mL NO₂ vapor dose at about 960Torr followed by a 2 min purge with argon, then a 6 mL TMA vapor dose atabout 10 Torr, followed by a 2 min purge. FIG. 4A is a plot of thethickness of the functionalization layer as a function of the number ofNO₂-TMA cycles.

As shown in the plot, the thickness of the functionalization layerremained constant at about one monolayer for up to about 100 NO₂-TMAcycles. This preserved functionalization layer thickness within thefirst 100 NO₂-TMA cycles confirms that the NO₂-TMA technique forms aself-terminating monolayer on the SWNT surface. When more than 100NO₂-TMA cycles were carried out, the functionalization layer increasedin thickness. Imperfections in the methyl surface eventually led toaddition of more material beyond the monolayer, which begins around 200cycles, and is prevalent by 400 cycles. This breakdown of theself-limiting behavior is found to cause a porous, nonuniformmulti-layer to form on the SWNT surface. Because fine control of coatingthickness and control of coating uniformity is a desired goal for manyapplications, it therefore can be preferred to use less than 100functionalization cycles.

Inertness of the surface methyl groups to chemical reaction with NO₂ maybe the reason that the functionalization layer is self-limited at amonolayer. Only when both water and TMA are added does a stabilizationlayer grow around the functionalization layer. Water molecules arelikely to cleave the methyl groups from their aluminum centers, leavinghydroxyl groups (—OH) in their place and methane (CH₄) as a gaseousbyproduct. This newly hydroxylated surface is in turn susceptible to ALDreactions through similar cleavage mechanisms.

EXAMPLE IV Functionalization Layer Thickness Control by NO₂ Exposure

SWNTs were synthesized as in Example I. The SWNTs were then subjected tothe functionalization process of Example III with the duration of theNO₂ dosing step in each NO₂-TMA functionalization cycles adjusted toelucidate the nature of the functionalization layer. FIG. 4B is a plotof functionalization layer thickness as a function of NO₂ dosing.

It is here shown that the thickness of the functionalization layer isdirectly proportional to the NO₂ exposure, i.e., the thickness dependson the number of collisions that NO₂ gas molecules can make with thefunctionalization layer before they are purged. More collisions increasethe chance of NO₂ molecules finding and adsorbing onto imperfections inthe layer. NO₂ adsorbed in this way can then act as an adsorption sitefor incoming TMA. As this happens over many cycles, a multi-layer withappreciable thickness can be produced. Based on this evidence, it can bepreferred to optimize the functionalization procedure of the inventionby using low exposure, flow-through style dosing, and a relatively lownumber of cycles, e.g., less than 100, as explained above.

EXAMPLE V Al₂O₃ Deposition on Functionalized SWNTs

A 200 nm-thick silicon nitride membrane was fabricated and FIB millingwas employed to form a through-hole extending through the membrane. Conanotube catalyst regions were formed by ALD of a Co layer on thenitride membrane surface and lift-off patterning of the layer, and thenthe SWNT growth of Example I was carried out with a 5 min growth time.This was sufficient to grow a relatively high yield of tubes that werelong enough to completely traverse the patterned FIB holes in themembrane. During the growth, SWNTs grew across the holes, making theirsuspended sections ideal for TEM analysis.

The suspended SWNTs were functionalized in accordance with the inventionby exposure to 50 NO₂-TMA ALD functionalization cycles in a vacuumpressure of about 300 mTorr, under the continuous flow of argon at aflow rate of about 50 sccm at a temperature of about 25° C. One cycleconsisted of a 30 mL NO₂ vapor dose at about 960 Torr followed by a 2min purge with argon, then a 6 mL TMA vapor dose at about 10 Torr,followed by a 2 min purge.

After the 50 functionalization cycles were complete, 5 cycles of astabilization process were carried out. Here the stabilization layer wasprovided as high-κ Al₂O₃ deposited by ALD at 25° C. One ALD Al₂O₃stabilization cycle consisted of a 6 mL does of TMA at 10 Torr, followedby a 2 min purge with argon, then water vapor at about 24 Torr pulsedfor 0.2 s in a vacuum at about 300 mTorr, followed by a 5 minute purgewith argon. A layer of about 0.5 nm in thickness was formed.

The temperature was then increased to 225° C. for additional Al₂O₃deposition on the functionalization and stabilization layers. This Al₂O₃deposition was carried by alternating doses of TMA vapor, at a flow ofabout 6 mL and a pressure of about 10 Torr, and water vapor, at apressure of about 24 Torr, with the valve open for about 0.2 s. Betweendoses, 30 to 60 second purges were carried out. The base pressure duringdeposition was 300 mTorr, and there was a constant flow of argon ofabout 50 sccm through the reactor during this process.

Each ALD cycle of Al₂O₃ was found to have added about 0.1 nm to theradius of the coating on the SWNT. 20 ALD cycles gave a coating with 2nm radial thickness. 100 ALD cycles gave a coating with a radialthickness of 10 nm. The resulting ALD coating around the SWNTs was foundto be uniform and continuous. This experimentally verifies thatdeposition of a selected insulating material on a stabilizedfunctionalization layer exhibits little if any inhibition or delay innucleation of the ALD growth.

EXAMPLE VI Prevention Against Schottky Barrier Modulation

SWNTs were synthesized as in Example I. The SWNTs were then exposed topulsed NO₂ doses. FIG. 5A is a plot of the measured conductance of aSWNT as a function of time as one NO₂ dose pulse was applied to the bareSWNT. The exposure caused a conductance increase of ˜10⁻⁵ A/V, followedby a decrease due to desorption, as explained in Example I.

After a 12 hour purge, the SWNT was coated with 10 nm of ALD Al₂O₃. OneALD Al₂O₃ deposition cycle consisted of a 6 mL does of TMA at 10 Torr,followed by a 2 min purge with argon, then water vapor at about 24 Torrpulsed for 0.2 s in a vacuum at about 300 mTorr, followed by a 5 minutepurge with argon. The process was carried at room temperature. This ALDdeposition did not coat the SWNTs, given the inert nature of the bareSWNT surface to ALD deposition, but did coat the Mo/SWNT junctions,passivating the Schottky barriers at the point between each end of aSWNT and the electrical contact on the surface. Without afunctionalization layer, the SWNTs were effectively self-masked fromdeposition of the ALD coating.

This experiment isolated SWNT conduction changes caused bycharge-transfer doping of the nanotubes from conduction changes causedby modulation of the Schottky barrier at the contacts between the SWNTsand the Mo electrodes. FIG. 5B is a plot of measured conductance as afunction of time as NO₂ dose pulses were applied to the SWNT after theMo/SWNT junctions were passivated. The conductance increase is now onlyabout 10⁻⁷ A/V, two orders of magnitude smaller than it was for SWNTshaving exposed Schottky barriers. From this it can be concluded thatmost of the conductance change caused by NO₂ occurs through modulationof the Schottky barriers. Accordingly, this further confirms that theNO₂ layer does not significantly perturb the electrical properties ofthe nanotube.

Further illustration of this condition was experimentally observedduring a NO₂-TMA functionalization treatment. One suspended SWNT withexposed Schottky barriers and one suspended SWNT having Schottkybarriers passivated by the Al₂O₃ deposition given above were bothexposed to 50 NO₂-TMA ALD functionalization cycles under the processconditions of Example III. FIG. 5C is a plot of conductance as afunction of time during the 50 cycle functionalization process for eachof the two SWNT conditions.

From the plotted data it is seen that the conductance of the SWNT havinguncoated Mo/SWNT contacts changed significantly while the conductance ofthe SWNT having coated Schottky barriers was relatively constant. Theseresults show that Schottky barrier modulation is the dominant mechanismcausing the conductance increase observed during functionalization. Thisis reasonable considering that NO₂ exposure is expected to alter metalwork functions. Furthermore, this demonstrates that thefunctionalization technique of the invention can be used withoutdramatically changing the conductance of a nanotube if the Schottkybarriers of the nanotube are adequately protected.

EXAMPLE VII Fabrication of Core-Shell Nanotube Structures

Suspended SWNTs having a 10 nm-thick Al₂O₃ coating overfunctionalization and stabilization layers were fabricated as in ExampleVI. The Al₂O₃-coated SWNTs were then coated with a selected metal layer.One particularly convenient metal deposition technique that can beemployed in accordance with the invention is the ALD coating processdescribed in U.S. Patent Application Publication No. US2006/0141155,published Jun. 29, 2006, entitled, “Atomic Layer Deposition Using MetalAmidinates,” by Gordon et al., the entirety of which is herebyincorporated by reference.

Metals including copper, cobalt, nickel, iron, ruthenium, manganese,chromium, vanadium, niobium, tantalum, titanium, lanthanum, rhodium, orother suitable metal layer can be formed by ALD techniques. For example,rhodium deposition by ALD can be carried out in accordance with theinvention as described by Aaltonen et al., “ALD of Rhodium Thin Filmsfrom Rh(acac)₃ and Oxygen,” Electrochemical and Solid-State Letts., V.8, N. 8, pp. C99-C101, 2005, the entirety of which is herebyincorporated by reference. Metal coating by evaporation, chemical vapordeposition, or other deposition technique can also be employed inaccordance with the invention to deposit gold, silver, palladium, oranother selected metal. Other suitable electrically conductingmaterials, such as tungsten nitride, can also be employed. One exampletechnique in accordance with the invention for depositing anelectrically conducting material such as tungsten nitride by ALD isdescribed by Becker et al., in “Highly Conformal Thin Films of TungstenNitride Prepared by Atomic Layer Deposition from a Novel Precursor,”Chem. Mater., N. 15, pp. 2969-2976, 2003, the entirety of which ishereby incorporated by reference.

Whatever deposition technique is employed, the metal deposition resultsin an insulator-metal core-shell structure around the SWNT. Thefunctionalization process of the invention enables this core-shellstructure to be formed with SWNTs, overcoming the inherent inertness ofSWNTs to conformal layer deposition. The functionalization process ofthe invention also enables this core-shell structure to be produced withALD processes that can control the thickness of the shell structure atthe nanometer scale and that can reliably and reproducibly provideconformal and uniform shell layers.

EXAMPLE VIII Hydrophilic Nanotube Coating

A carbon nanotube functionalized in accordance with the invention as inthe examples above is coated with a selected one or more materials torender the carbon nanotube hydrophilic and therefore amenable tosuspension in a polar solvent such as water or alcohol. Suspensions ofsuch hydrophilic nanotubes can then be employed, e.g., in medicalapplications. It is recognized that hydrophilic carbon nanotubes areless toxic than uncoated nanotubes and therefore are safer for medicalapplications. The nanotube hydrophilic conditions render the nanotubewell-suited for a wide range of applications beyond medical applicationsin which it can be desirable to process nanotubes in a polar solventsuch as water.

A hydrophilic layer such as a metal oxide or a silicon dioxide layer isdeposited by a selected technique, e.g., ALD, on a functionalized carbonnanotube for rendering the nanotube hydrophilic. One example ALDtechnique in accordance with the invention for deposition of silicondioxide or any in a range of metal oxides is described in U.S. Pat. No.6,969,539, issued Nov. 29, 2005, by Gordon et al., entitled “VaporDeposition of Metal Oxides, Silicates and Phosphates, and SiliconDioxide,” the entirety of which is hereby incorporated by reference.Chemical vapor deposition (CVD), including low pressure chemical vapordeposition (LPCVD), plasma enhanced CVD (PECVD), physical vapordeposition (PVD), or other selected technique can be employed to formthe oxide layer. Additionally, a metal layer provided on afunctionalization nanotube can itself be oxidized to form an oxidelayer.

EXAMPLE IX Fabrication of Coaxial Nanotubular Structures

Carbon nanotubes are synthesized and coated with functionalization andstabilization layers as in Example III. The coated carbon nanotubes arethen coaxially coated with one more layers of material that preferablyare oxidation resistant, e.g., with the Al₂O₃ material as in Example VI.The coated carbon nanotube structure is then separated from the surfaceor surfaces from which it was synthesized, to produce free carbonnanotubes and to expose the nanotube ends. In one example of such atechnique, the nanotube structure and support arrangement is suspendedin a selected liquid medium, e.g., alcohol, and ultrasonicated to breakthe ends of nanotubes from the support structure. This releases thecoated nanotubes from the support structure and suspends the nanotubesin the medium. The liquid medium is then evaporated and the nanotubesare dried.

The carbon nanotube itself is then removed from each coated nanotubestructure by any suitable method, e.g., by oxidation. When heated in anoxidizing atmosphere, such as air or oxygen, to a sufficiently hightemperature, such as over about 600° C., carbon nanotubes oxidize andcan be completely annihilated by oxidation at, e.g., 700° C.-800° C. for2 hours. Annihilation of the carbon nanotube from the coating structureproduces a hollow center surrounded by coaxial layers of the materialthat was deposited on the nanotube. As a result, a nanotubular structurehaving an inner diameter that corresponds to the diameter of theoriginal carbon nanotube is formed.

The original carbon nanotube thereby operates as a removable scaffold,and its removal produces precisely-controlled hollow nanotubes of theselected deposited material or materials. The uniform, conformal natureof material deposition that is enabled by the functionalization processof the invention allows for this hollow nanotube production process. Byemploying an ALD technique to form the coaxial material layers, thediameters of the resulting tubes to be produced by the depositedmaterial can be set at any desired value because growth per ALD cycle ishighly reproducible. By changing reactants after a selected number ofALD cycles, coaxial core-shell structures can also be constructed. Anynumber of layer arrangements can be employed in formation of such astructure, providing, e.g., conducting, insulating, and semiconductinglayer arrangements for use in electronic and sensing applications.

In one example, a hollow nanotubular structure thusly produced can beemployed as a hollow gas sensing structure. It is recognized that theelectrical resistance of semiconducting metal oxides, such as tin oxideand cobalt oxide, is sensitive to various gases. A metal oxidenanotubular structure therefore enables the production of a highlysensitive gas sensor.

For many applications, the hollow nanotubular structures must be placedbetween electrodes or other device elements for making electricalconnection in a circuit or other system. Such an arrangement can beaccomplished by, e.g., suspending the nanotubular structures in asolvent and spinning the solution onto a surface having electrodesprovided thereon. Alternatively, lithography can be used to formelectrodes at selected locations once the nanotubular structures arepositioned on a selected surface.

Considering this arrangement for a hollow gas sensing structure, thenanotubular structure can be, e.g., tin oxide or cobalt oxide. Theconductance of the nanotube can be monitored by flowing current throughthe nanotube. When the nanotubular gas sensor is exposed to gas, theenergy levels in the nanotube are altered, causing a change in thenanotube conductance. A transistor configuration is not required for agas sensing device as no gate is necessary.

A hollow nanotubular structure can also be employed as a nanowaveguidewith appropriate core and cladding material selection. One example ofsuch materials are described by Govyadinov et al., “Gain-Assisted Slowto Superluminal Group Velocity Manipulation in Nanowaveguides,” Phys.Rev. Lett., V. 97, p, 223902++.

EXAMPLE X Nanotube-Based Dielectric Etch Mask

The formation of nanotubular structures enables a wide range ofadditional applications and processes. In one such example process, anetch mask is formed of nanotubular structures. A material that issubstantially impervious, or at least highly resistant to, a particularetch species is layered on a functionalized carbon nanotube forproducing an etch mask material. In this process, a carbon nanotubesurface is first functionalized and stabilized as in Example III above.Then a selected etch masking material is deposited on the functionalizednanotube to form a uniform coaxial layer around the nanotube surface.

If desired for a given application, the carbon nanotube is then removedfrom the structure, following the techniques of Example IX above, toproduce a nanotubular structure of the masking material. Alternatively,the carbon nanotube is retained in the nanotubular structure. A liquidsuspension of masking material nanotubes is then produced by placing thetubes in a liquid medium such as water. The liquid suspension of maskingmaterial nanotubes forms a liquid nanotube resist that can bespin-coated onto a surface in the conventional manner. Once spin-coatedon a surface, various etch process techniques can be carried outemploying the nanotube resist layer as a masking layer. After completingthe etching, the nanotube resist layer can be removed by a suitabletechnique, e.g., wet etching, dry etching, or chemo-mechanicalpolishing.

In one example of this nanotube masking process, nanotube coatingmaterials are selected specifically to enable masked etching of adielectric material on a substrate. Given a fluorine-based plasma recipefor dielectric etching, as is conventional, then a material such asSc₂O₃, Y₂O₃, LaAlO₃, MgO, GdScO₃, or other selected material can beemployed as the nanotubular mask material to be coated on a nanotube.One suitable deposition process for Sc₂O₃ coating in accordance with theinvention is described in “ALD of Scandium Oxide from Scandiumtris(N,N-diisopropylacetamidinate) [Sc(amd)₃] and Water,” by Gordon etal., Electrochemical and Solid-State Letters, V. 9 N. 6, pp. F45-F48,2006, the entirety of which is hereby incorporated by reference. U.S.Patent Application Publication No. 2006/0141155 cited above alsodescribes suitable deposition techniques. One suitable depositionprocess for GdScO₃ coating in accordance with the invention is describedby Kim et al., in “Atomic layer deposition of gadolinium scandate filmswith high dielectric constant and low leakage current,” Appl. Phys.Lett., V. 89, pp. 133512-1-133512-3, 2006, the entirety of which ishereby incorporated by reference. One suitable deposition process forLa₂O₃ coating in accordance with the invention is described by Lim etal., in “Atomic layer deposition of lanthanum aluminum oxidenano-laminates for electrical applications,” Appl. Phys. Lett., V. 84,N. 20, pp. 3957-3959, May 2004, the entirety of which is herebyincorporated by reference. Deposition of a layer of one of the selectedmaterials having a thickness of, e.g., about 5 nm-10 nm, is carried outby ALD or other suitable technique on functionalized carbon nanotubes.

The carbon nanotubes can then be annihilated by the oxidation process ofExample IX, resulting in nanotubular structures of the selected oxidematerial, or can be retained in the structures if desired. A suspensionof the nanotubular structures can then be formed with a selected liquidmedium. This liquid suspension is spin-coated on a dielectric layer tobe etched by a fluorine-based plasma. Scandium oxide etches 100 to 1000times slower than silicon dioxide and therefore is an excellent etchmask for silicon dioxide etch processes.

In one example etch process provided by the invention, the oxidenanotube etch mask is spin-coated on a substrate to provide a blanketmasking layer over a dielectric layer on the substrate, for which alow-κ characteristic is desired. Such can be the case for, e.g.,intermetal dielectric layers where a dielectric constant of less thanabout 4 is desired. The spin-coated masking layer includes oxidenanotubes at random orientations distributed across the dielectric layersurface.

With the masking layer in place, a fluorine-based plasma etch process iscarried out: 200 W of microwave power, 50V of DC bias, 2.0 sccm O₂ flowrate, 40.0 sccm CF₄ flow rate, and a pressure of 20.0 mTorr. The randompattern of the layer of masking oxide nanotubes shields the underlyingdielectric layer in a corresponding fashion such that the plasma etchforms vertical walls, through the layer, of the protected materialregions, that are oriented in random directions across the layer. Thehigh-aspect ratio pattern of oxide nanotube distribution in the maskinglayer is therefore replicated in the underlying etched dielectric layerby the plasma etch. This etch process results in a porous dielectriclayer having a commensurate low-κ characteristic. The dielectricconstant of the dielectric layer can be prespecified, e.g., as less thanabout 4, and tailored by correspondingly selecting the density ofnanotubular structures included in the spin-coated masking layer. It isto be recognized that some empirical analysis can be required tocorrelate the oxide nanotube density of a masking layer with aprescribed dielectric constant for a dielectric layer to be etched.

The walls in the dielectric layer formed by the plasma etch process areconsiderably more robust than cylindrical pillars of comparabledimensions made from nanoparticle masks. As a result, this methodenables chemomechanical polishing (CMP) to be carried out on soliddielectric layers that have high mechanical strength, rather than onconventional porous dielectrics that have low strength. Only after theCMP of a top layer is complete is the dielectric etched. In addition,the random orientation of nanotubes produces randomly oriented walls ofthe remaining dielectric layer enabling a high shear strength in anydirection.

In a typical interconnect structure with trenches and vias underneath,the etch depth may conveniently extend at least through the thicknesscorresponding to the trenches, because the capacitance between thetrenches causes more significant delays in signal propagation than thedielectric remaining between the vias. Once the porous etch method iscomplete, the etch mask can be removed by any convenient cleaningprocess, such as blowing with a gas or a spray of dry ice pellets, or byCMP. Any nanotubes that escape the polishing step to remain on thedielectric layer are harmless in that they are insulators themselves andtherefore cannot cause a short circuit between metal layers.

EXAMPLE XI Fabrication of a Coaxially-Gated Ballistic SWNT Field EffectTransistor

A SWNT is synthesized in a configuration that is suitable for coaxialcoating, such as the synthesis process of Example I. The surface of thesynthesized, suspended SWNT is then functionalized, stabilized, andcoaxially coated with an insulator, such as Al₂O₃, and then a selectedmetal, such as WN, as in Example VII. The coaxial, suspended core-shellSWNT structure is then coated with a suitable photoresist and thephotoresist is exposed only at the ends of the SWNT structure with UVlight, electrons, or other suitable resist exposure species. The resistis then developed, exposing only the ends of the SWNT core-shellstructure.

The metal and insulator materials are then etched off of the surface ofthe SWNT at the two ends of the nanotube, e.g., by conventional metaland oxide wet or dry etches, to expose the surface of the nanotube atits ends. Electrical contact pads and electrical connections to the SWNTsurface then are formed. The contact pads and electrical connections areproduced by evaporation of Pd, Rh or other selected metal that does notform a Schottky barrier with the nanotube onto this structure, makingelectrical contact between with the exposed SWNT ends and contact pads.A photoresist lift-off technique is employed to pattern the metalcontact material. The remaining resist is removed with conventionallift-off procedures, producing a SWNT that is coaxially passivated withan insulator, coaxially gated with a metal, and electrically accessiblethrough the metal contacts at the ends of the nanotube. This structureprovides a SWNT field effect transistor geometry that is very preciselydefined and fabricated by the ALD processes of the invention. Withpalladium or rhodium, or other similar metal as the electricallyconducting contact material, the structure is characterized as aballistic field effect transistor.

EXAMPLE XII Coaxially-Gated Schottky Barrier-Modulated SWNT Field EffectTransistor

Field effect transistor fabrication with a suspended SWNT is carried outas in Example XI. But instead of Pd or Rh, a metal that produces aSchottky barrier at each metal/SWNT junction, such at Mo or Pt, isemployed to make contact to the ends of the nanotube. The result is aSWNT field-effect transistor that is modulated by carrier transport overand through the Schottky barriers.

EXAMPLE XIII Bulk-Modulated SWNT Field Effect Transistor

Referring to FIG. 6A (not to scale), a supporting substrate 50, e.g., aquartz chip, or other suitable substrate is provided. Electricallyconducting contact pads 52, 54, of, e.g., 50 nm-thick Mo and 50 nm-thickTi layers, are patterned on the substrate to provide transistor sourceand drain connections. A metal line 56 of Mo and Ti is provided betweenthe source and drain pads 52, 54. The source and drain pads and metalline are formed by conventional lift-off.

Referring to FIG. 6B, then, as in Example I, a trench 62 of 1 μm inwidth and 0.5 μm in depth is cut, by focused ion beam or other suitablemethod, into the substrate 50, across the metal line 56 between thesource and drain pads. This forms a gap 60 between the source and drainpads and self-aligns the edges of the source and drain pads with theedge of the trench 62. As shown in FIG. 6C, 0.5 nm-thick layer 64 of,e.g., Al₂O₃ or other selected material is then deposited by ALD on theelectrode surfaces to prevent alloying between the molybdenum and thenanotube catalyst metal.

Referring to FIG. 6D, then a catalyst layer 66 of Co of about 0.3 nm inthickness is deposited and patterned by liftoff on the electrodesurfaces to form catalyst regions. With the catalyst in place, then asin Example I above, one or more SWNTs 70 are synthesized across thetrench 60 between source and drain, as shown in FIG. 6E. The catalystagglomerates at the synthesis temperature and is effectively removed bythis process. For clarity only a single synthesized nanotube is shown inthe figures.

Turning to FIG. 6F, a portion of the source and drain electrodes 52, 54far from the suspended SWNT are masked with glass slide 74 or similarsmooth surface, or with a photolithography resist. The structure is thencoated with a layer 72 of Al₂O₃ or other suitable material. Thisinsulating layer 72 coats the pre-patterned metal electrodes, except theregion underneath the masks 74. The insulating layer does not coat thesuspended section of the SWNT because as-synthesized, the nanotube isinert to vapor deposition processes such as the ALD deposition process.The nanotube is thereby self-masked from the insulator depositionprocess. The insulating layer does, however, coat the junction at theends of the SWNT and the source and drain pads because the dielectriclayer can grow up from the substrate surface around the edges of thenanotube. This selective deposition can be carried out with any selectedvapor process, including, e.g., low-temperature CVD processes, to whicha SWNT is chemically inert.

The selective nature of the deposition during this insulating layerformation enables the precise definition of the nanotube gate length.Specifically, the thickness of the layer can be selected to define achosen gate length. Given that the suspended nanotube length can beapproximated as the width of the trench prior to the nanotube synthesisstep, then the Al₂O₃ thickness can be correspondingly selected so thatthe extent of the nanotube surface that is left uncoated by this stepdefines the gate length. For example, given a 200 nm-wide trench, then aAl₂O₃ layer of 50 nm in thickness coats 50 nm at the edges of thetrench, as shown in FIG. 6F, leaving an uncoated nanotube suspensionlength of 100 nm. This 100 nm nanotube length then defines the length ofthe gate.

With this insulating layer in place, the SWNT is then functionalized asin Example III, with a functionalization and a stabilization layerformed on the nanotube surface. Referring to FIG. 6G, then a gatedielectric layer 76 of Al₂O₃ is deposited by ALD or other suitabletechnique. Here the deposition does coat the functionalized suspendednanotube to define a suspended gate the length of which is defined bythis coating. The gate dielectric layer 76 is therefore preferably of athickness selected as the coaxial gate oxide thickness for the fieldeffect transistor. For many applications, a gate oxide thickness ofabout 5 nm can be preferable. Thicknesses less than about 5 nm canenable electronic tunneling phenomena, and thicknesses greater thanabout 5 nm can result in a loss of capacitive coupling with the nanotubegate.

Referring to FIGS. 6H-6I, a coaxial metal gate layer 78 of WN, TiN, Cu,Ru, or other material is then deposited and patterned by the ALDtechniques of the examples above, and with lift-off over the SWNTlength. FIG. 6I is a schematic planar view, not to scale, indicating thegeometry of the gate layer 78 across the length of the nanotube 70 andoverlapping the ends of the source and drain connections 52, 54 with thenanotube. The thickness of the gate metal is preferably sufficientlylarge to produce a sheet resistance that is sufficiently small for theselected device geometry. ALD of WN is a preferred gate material with athickness greater than about 20 nm.

With this configuration, the thickness of the insulating layer at thejunction of the SWNT and the source and drain electrical connections isgreater than the thickness of the gate dielectric encasing thefunctionalized length of the SWNT. As explained above, this results fromthe insulating layer deposition conditions under which theunfunctionalized nanotube is inert to the deposition, and preciselydefines the length of the nanotube that operates as the gate. With thisarrangement, the electric field generated by application of a voltage tothe gate electrode has a greater effect along the thinly-coated SWNTlength than at the thickly-coated SWNT/metal junctions at the ends ofthe nanotube. Given that the insulating layer at the SWNT/metaljunctions is sufficiently thick, the effect of the field at thosejunctions is substantially negligible, and the SWNT field effecttransistor is well-controlled for bulk-modulated operation.

EXAMPLE XIV Vertical SWNT Field Effect Transistor

Referring to FIG. 7A (not to scale), a supporting substrate 100, e.g., aquartz chip, or other suitable substrate is provided. A trench 102 isformed in the substrate and a catalyst region 104 of Co is formed in thetrench for growth of a nanotube in the trench. The metal catalyst region104 is produced by evaporation and lift-off, as in the examples above.Alternatively, an isolated catalyst region 104 can be produced byblanket ALD of Co or other metal over the entire structure and polish ofthe metal off of the top surface of the substrate 100. A non-conformalrefractory protective layer could instead be deposited over the catalyston the surface of the substrate 100. For example, CVD or non-conformal(low-exposure) ALD SiO₂ may be used to cover and de-activate thecatalyst on the top surface, where nanotubes are not desired.

Referring to FIG. 7B, a carbon nanotube 110 is then grown following thesynthesis of Example I vertically up and extending out of the trench102. As shown in FIG. 7C, a layer 112 of metal is then deposited overthe structure by ALD techniques in the manner given in the examplesabove for, e.g., Rh, Pd, and Ru. Because an ALD technique is employedfor the metal deposition, the section 114 of the nanotube 110 that isfreely standing above the substrate is not coated with the metal, giventhe inertness of the nanotube to the ALD process. The region of thenanotube in the trench 102 is coated with the metal because the metalgrows from the walls of the trench and thus deposits around the nanotubein the trench. This forms the source (or drain) of the transistor withan electrical connection to the nanotube.

Then as shown in FIG. 7D an insulating layer 116 of silicon dioxide isdeposited over the structure, by a vapor process such as a CVD processor the ALD techniques in the manner of the examples above. The selectedvapor process preferably is one to which the SWNT is inert. With thiscondition, the section 118 of the nanotube 110 that is freely standingabove the substrate is not coated with the insulating layer, given theinertness of the nanotube to the ALD or other selected vapor process.

The surface 120 of the nanotube 110 is then functionalized andstabilized following the procedure of the examples above, as shown inFIG. 7E, and then as shown in FIG. 7F, a layer 122 of gate dielectric isdeposited by ALD techniques over the structure. This adds to thethickness of the first insulating layer 116 and coats the functionalizedcarbon nanotube 110 as well. Referring to FIG. 7G, a layer 124 of gatemetal is then deposited by ALD techniques in the manner of the examplesabove, coating the gate oxide 122 over the nanotube and coating thethicker oxide layer 116.

Referring to FIG. 7H, a layer 126 of silicon dioxide is then depositedby ALD in the manner of the examples above, covering the gate metallayer 124 for defining the connection to the drain region of thetransistor. Then as shown in FIG. 7I, the top layers are polished off byCMP to expose the end 130 of the nanotube 110. Then as shown in FIG. 7J,the top region 134 of the gate insulator layer 122 is removed fromaround the nanotube. This is carried out by a chemically selective etchthat removes the high-k oxide without dissolving the thicker insulator126. For example, if the insulator 126 is SiO₂, then hydrochloric acidcan be used to selectively remove a high-k insulator, such as lanthanumoxide.

As shown in FIG. 7K, the top region 136 of the metal gate layer 124 isthen selectively removed. For example, if tungsten nitride is used as agate metal, then it is selectively removed by a hydrogenperoxide-ammonia etch solution. With this configuration, then, as shownin FIG. 7L, a layer 140 of silicon dioxide is deposited by ALDtechniques on the structure. The silicon dioxide deposits around thenanotube 110 in a region 142 where other surfaces are available fordeposition, but does not deposit on the exposed nanotube itself, due tothe inert nature of the bare nanotube surface to the ALD process.Finally, as shown in FIG. 7M, a metal layer 144 is deposited by ALD toform the drain contact. With this deposition, source and drain contacts112, 144, and a gate contact 124 are provided for connection along thenanotube transistor structure, and bulk modulation of the transistoralong the nanotube length is achieved.

EXAMPLE XV Functionalization of Silicon

A p-type Si wafer was functionalized in accordance with the invention.Before functionalization, the Si substrate, of 0.05-0.07Ω·cm, was dippedin dilute hydrofluoric acid (HF) to remove the native oxide. Thesubstrate was then exposed to 5 min of UV/ozone processing to removesurface organics, and re-dipped in dilute HF to remove any oxide createdfrom ozone exposure. The HF-last Si wafer was then functionalized underoptimized conditions of 50 NO₂-TMA cycles at a pressure of 300 mTorr anda temperature of 25° C., and coated with 10 nm of ALD Al₂O₃ at 250° C.For comparison, a 10 nm-thick layer of ALD Al₂O₃ was also formed on aclean but non-functionalized p-type Si wafer.

MOS capacitors were then fabricated on the functionalized andnon-functionalized p-type Si substrates. 50-nm-thick, 1×10⁻⁴ cm²aluminum control gates were photolithographically patterned on the topsurface of the 10-nm-thick ALD oxide layer. Because the back surfaces ofthe substrates were also coated with ALD oxide during the oxidedeposition, the backside Al₂O₃ layer was etched with phosphoric acid(H₃PO₄), and the underlying native SiO₂ was etched with dilute HF toexpose the silicon underneath. A 50 nm-thick layer of Al was thendeposited by evaporation on the backside to form a second contactelectrode and thus to complete MOS capacitor fabrication.

Capacitance-Voltage (C-V) analysis was performed on the functionalizedand non-functionalized MOS capacitors. FIG. 8A is a plot of capacitanceas a function of voltage for a non-functionalized Si substrate. FIG. 8Bis a plot of capacitance as a function of voltage for anon-functionalized Si substrate. Capacitors formed on the substratewithout the functionalization layer exhibited small amounts ofhysteresis and negligible frequency dispersion. Capacitors formed on thesubstrate with functionalization exhibited significant hysteresis andfrequency dispersion.

From this data, it is evident that the NO₂-TMA functional layer degradedthe quality of the MOS dielectric. The pronounced counterclockwisehysteresis and significant flat-band voltage frequency dispersion of thefunctionalized structure is indicative of a dielectric containingtrapped dipoles. This is understood to result from the polar nature ofNO₂.

EXAMPLE XVI Annealing of Functionalized Silicon

P-type Si wafers like those of Example XV were functionalized in themanner of Example XV and were subjected to an anneal prior tometallization in the manner of Example XV. Six different annealtemperatures were employed for six distinct anneal processes. Allanneals were performed in a 200 sccm flowing Ar ambient for two hours atatmospheric pressure. The electrode patterning of Example XV was thencarried out.

FIGS. 9A-9F are plots of characteristic hysteresis and frequencydispersion for measured C-V curves for Si wafers having no anneal, ananneal at 300° C., at 400° C., at 500° C., at 600° C., and at 700° C.,respectively. It is seen from the plots that the hysteresis andfrequency dispersion decrease with higher annealing temperatures. Theoptimal temperatures are found to be between 500° C. and 600° C., whereboth the hysteresis and the frequency dispersion are relativelynegligible. This is also true at 700° C., however the flat-band voltageshifts away from 0 V at this anneal temperature, and therefore thistemperature may not be preferable for all applications.

Ellipsometry was carried out on an Al₂O₃/NO₂-TMA film prior to theannealing step and after the annealing step. It was found that thefunctionalization layer adds an additional 3 nm-3.5 nm of thickness tothe MOS structure. This thickness is not changed by the annealingprocess. It is understood that this measurement indicates thatappreciable amounts of the functional complexes do not diffuse out ofthe dielectric, and suggests that the improved C-V characteristicsproduced by an anneal are the result of another NO₂ passivationmechanism.

To determine further the nature of the functionalization layer impact oncapacitive characteristics, the MOS structure of Example XV was againfabricated on a p-type Si wafer, of 14-20Ω·cm, but here with a 2nm-thick layer of SiO₂ provided on the wafer. The functionalizationlayer was sandwiched between the SiO₂ layer and a capping layer of 10nm-thick Al₂O₃.

The C-V characteristic of this MOS structure was measured. Significanthysteresis and frequency dispersion were measured even for this examplewith the functional layer embedded inside the dielectric as opposed todeposited directly on the Si surface. This configuration eliminated anyinteraction between the functionalization layer and the Si interface,and therefore reinforces the conclusion that the functionalization layeris the direct cause of the observed capacitance characteristics.

As a result of these measurements, a suitable anneal, e.g., a 500° C.anneal, after functionalization and ALD dielectric coating, can bepreferred in accordance with the invention. Such an anneal does notalter the breakdown field of the insulator, but can cause some increasein the leakage current through the film. Even with such an increase, itis preferred in accordance with the invention to include an annealingstep for processes in which non-ideal capacitive characteristics aredetrimental for a given application.

EXAMPLE XVII Functionalization and Formation of Dielectric Layers onGraphite and Graphene

A graphite sheet and a graphene layer are individually subjected to 50NO₂-TMA ALD functionalization cycles in a vacuum pressure of about 300mTorr, under the continuous flow of argon at a flow rate of about 50sccm at a temperature of about 25° C. One cycle consists of a 30 mL NO₂vapor dose at about 960 Torr followed by a 2 min purge with argon, thena 6 mL TMA vapor dose at about 10 Torr, followed by a 2 min purge.

After the 50 functionalization cycles are complete, 5 cycles of astabilization process are carried out. Here the stabilization layer isprovided as high-κ Al₂O₃ deposited by ALD at 25° C. One ALD Al₂O₃stabilization cycle consists of a 6 mL does of TMA at 10 Torr, followedby a 2 min purge with argon, then water vapor at about 24 Torr pulsedfor 0.2 s in a vacuum at about 300 mTorr, followed by a 5 minute purgewith argon. A layer of about 0.5 nm in thickness is formed.

The temperature is then increased to 225° C. for additional Al₂O₃deposition on the functionalization and stabilization layers. This Al₂O₃deposition is carried by alternating doses of TMA vapor, at a flow ofabout 6 mL and a pressure of about 10 Torr, and water vapor, at apressure of about 24 Torr, with the valve open for about 0.2 s. Betweendoses, 30 to 60 second purges are carried out. The base pressure duringdeposition is 300 mTorr, and there is a constant flow of argon of about50 sccm through the reactor during this process.

An anneal is carried out as follows. The base pressure is increased to 1atm under 200 sccm of flowing Ar. Once at 1 atm, the Ar flow is kept at200 sccm, flowing through the chamber and out through an oil bubbler.The furnace is then ramped to 500° C., and maintained at thistemperature for 2 hours. After 2 hours, the temperature is decreased to25° C., still under 200 sccm Ar at 1 atm. If desired, the structure isthen ready for further processing.

The resulting thin layer of aluminum oxide can be employed on thegraphite sheet and on the graphene layer to electrically insulate partor all of the graphite or graphene surface as part of a microelectronicdevice or structure. Further, the metal oxide layer can be employed toprotect the graphite and graphene from oxidation at high temperatures,or to protect the graphite and graphene from highly oxidizing chemicalenvironments.

In further techniques, hafnium oxide, rather than aluminum oxide, orother material can be deposited on the functionalized graphite and onthe graphene layers, e.g., as described by Hausmann et al., Chemistry ofMaterials, V. 14, pp. 4350 to 4358, 2002, herein incorporated byreference.

EXAMPLE XVIII Anneal of Functionalized Carbon Nanotubes

Carbon nanotubes that were functionalized as in Example III are placedin a furnace chamber and the base pressure is increased to 1 atm under200 sccm of flowing Ar. Once at 1 atm, the Ar flow is kept at 200 sccm,flowing through the chamber and out through an oil bubbler. The furnaceis then ramped to 500° C., and maintained at this temperature for 2hours. After 2 hours, the temperature is decreased to 25° C., stillunder 200 sccm Ar at 1 atm.

The functionalization, stabilization, and annealing steps of the presentinvention, as described above, can be applied to a wide range ofsubstrates and three-dimensional structures, such as carbon nanotubes,other fullerene structures, planar and non-planar carbon-basedmaterials, and various microelectronic structures. The invention iswell-suited for enabling material deposition on any three-dimensionalstructure, planar or non-planar, that is inherently inert to suchdeposition and that is compatible with the functionalization andstabilization processes.

Thus, once the functionalization and stabilization steps have beencompleted on a selected structure and an anneal step completed ifdesired for a given application, the structure can be used as a scaffoldfor growing many other materials by ALD or other selected process,including metals and their oxides, nitrides, and sulfides. With ALD asthe material coating process, the thickness of the coating can be set atany desired value because the growth per ALD cycle is highlyreproducible. By changing reactants after a certain number of ALDcycles, nanolaminate or core-shell structures can also be constructed,e.g., with nanotubes. Thin plates of oxidation-resistant material can beformed by oxidation of a graphite substrate under which anoxidation-resistant coating is provided after the functionalizationprocess.

For many applications, an anneal step can be appropriate to reduce oreliminate undesirable capacitive properties, including trapped chargesand recombination centers. Because the functionalization technique ofthe invention avoids covalent modification of the underlying structure,it is understood that the optoelectronic properties of structures arepreserved. A wide range of electronic devices, including SWNT deviceswith ultrathin gate dielectrics and coaxially-gated geometries, andplanar graphene devices, are enabled by the processes of the invention.A wide range of applications of the functionalized carbon nanotubesprovided by the invention include coaxially-gated carbon nanotube-baseddevices, carbon nanotube-based sensors, and nanotubes of selectedmaterials for, e.g., etch masking.

By using optimized functionalization parameters, graphene, graphite,nanotubes, and other structures can be coated with high-κ dielectricmaterial that is exceptionally thin, continuous, and conformal. Changesin material conductance caused by functionalization can be avoided bySchottky barrier passivation. It is recognized, of course, that thoseskilled in the art may make various modifications and additions to theprocesses of the invention without departing from the spirit and scopeof the present contribution to the art. Accordingly, it is to beunderstood that the protection sought to be afforded hereby should bedeemed to extend to the subject matter of the claims and all equivalentsthereof fairly within the scope of the invention.

We claim:
 1. A method for functionalizing a graphene layer comprising:exposing the graphene layer to at least one vapor including at least onefunctionalization species, selected from the group consisting of NO₂ andCH₃ONO, that non-covalently bonds to the graphene layer while providinga functionalization layer of chemically functional groups, to produce afunctionalized graphene layer; and exposing the graphene layer to atleast one vapor stabilization species to form a stabilization layer thatstabilizes the functionalization layer against desorption from thegraphene layer while providing chemically functional groups at thegraphene layer, to produce a stabilized graphene layer.
 2. The method ofclaim 1 wherein the graphene layer is disposed on a substrate.
 3. Themethod of claim 1 wherein the graphene layer is disposed on a layer ofoxide.
 4. The method of claim 1 wherein the chemically functional groupscomprise —CH₃ groups.
 5. The method of claim 1 wherein the at least onevapor comprises a first vapor including a first functionalizationspecies that is physisorbed on the graphene layer and a second vaporincluding a second functionalization species that reacts with thephysisorbed first functionalization species to form on the graphenelayer a functionalization layer of chemically functional groups.
 6. Themethod of claim 5 wherein the second functionalization species isselected from a group consisting of trimethylaluminum, dimethylzinc,trimethylgallium, trimethylindium, trimethylbismuth,tetrakis(dimethylamido)hafnium, and tetrakis(dimethylamido)zirconium. 7.The method of claim 5 wherein the graphene layer exposure comprisescyclically alternating exposure of the graphene layer to the first vaporand the second vapor.
 8. The method of claim 1 wherein thefunctionalization species exposure of the graphene layer and thestabilization species exposure of the graphene layer are carried out ata temperature of about 25° C.
 9. The method of claim 1 furthercomprising exposing the stabilized graphene layer to at least onematerial layer precursor species that deposits a material layer on thestabilized the graphene layer.
 10. The method of claim 7 wherein thecyclically alternating exposure comprises cycles of atomic layerdeposition with the first functionalization species and the secondfunctionalization species.
 11. The method of claim 7 wherein thecyclically alternating exposure is carried out a number of exposurecycles that forms a functionalization layer of no greater than onemonolayer in thickness.
 12. The method of claim 7 wherein the cyclicallyalternating exposure is carried out a number of exposure cycles thatforms a functionalization layer of no greater than about one nanometerin thickness.
 13. The method of claim 1 wherein the stabilizationspecies reacts with the functionalization layer to covalently bond tothe functionalization layer.
 14. The method of claim 1 wherein the atleast one vapor stabilization species consists of water and a speciesselected from a group consisting of trimethylaluminum, dimethylzinc,trimethylgallium, trimethylindium, trimethylbismuth,tetrakis(dimethylamido)hafnium, and tetrakis(dimethylamido)zirconium.15. The method of claim 1 wherein the stabilization layer that is formedcomprises Al₂O₃.
 16. The method of claim 1 wherein the vaporstabilization species exposure of the functionalized graphene layercomprises cyclically alternating exposure of the functionalized graphenelayer to a first stabilization precursor and a second stabilizationprecursor.
 17. The method of claim 16 wherein the cyclically alternatingexposure comprises cycles of atomic layer deposition with the firststabilization precursor and the second stabilization precursor.
 18. Amethod for functionalizing a graphene layer comprising: exposing thegraphene layer to at least one vapor including at least onefunctionalization species that non-covalently bonds to the graphenelayer while providing a functionalization layer of chemically functionalgroups, to produce a functionalized graphene layer; exposing thefunctionalized graphene layer to at least one vapor stabilizationspecies to form a stabilization layer that stabilizes thefunctionalization layer against desorption from the graphene layer whileproviding chemically functional groups at the graphene layer, to producea stabilized graphene layer; and forming an electrically insulatinglayer over the stabilization layer.
 19. The method of claim 18 whereinthe electrically insulating layer that is formed over the stabilizationlayer is selected from a group consisting of Al₂O₃, LaAlO₃, HfO₂, ZrO₂,Ta₂O₅, and mixtures thereof.
 20. The method of claim 18 furthercomprising forming an electrically conducting layer over the insulatinglayer.
 21. The method of claim 20 wherein the electrically conductinglayer is selected from a group consisting of Rh, Pd, WN, and TiN. 22.The method of claim 18 wherein the graphene layer is disposed on asubstrate.
 23. The method of claim 18 wherein the graphene layer isdisposed on a layer of oxide.
 24. The method of claim 18 wherein thestabilization layer that is formed comprises Al₂O₃.
 25. The method ofclaim 18 wherein the chemically functional groups comprise —CH₃ groups.